Part 3

Fig. 9. Longitudinal section through the notochord and adjoining parts to shew the first appearance of the cartilaginous notochordal sheath which forms the vertebral centra. Stage N.

Fig. 10. Transverse section through the tail of an embryo of stage P to shew the coexistence of the rib-process and hæmal arches in the first few sections behind the point where the latter appear. Zeiss C, ocul. 1.

Fig. 11. Transverse section through the centre of a caudal vertebra of an embryo somewhat older than Q. It shews (1) the similarity between the arch-tissue and the hyaline tissue of the outer layer of the vertebral centrum, and (2) the separation of the two by the membrana elastica externa[2] (_mel_). It shews also the differentiation of three layers in the vertebral centrum: vide p. 374.

Footnote 2: The slight difference observable between these two tissues in the arrangement of their nuclei has been much exaggerated by the engraver.

EXPLANATION OF PLATE 14. (X. p. 378.)

_This Plate illustrates the Formation of the Spinal Nerves._

COMPLETE LIST OF REFERENCE LETTERS.

_ar._ Anterior root of a spinal nerve. _ch._ Notochord. _com._ Commissure connecting the posterior roots of the spinal nerves. _i._ Mesoblastic investment of spinal cord. _mp._ Muscle-plate. _n._ Spinal nerve. _nc._ Neural canal. _pr._ Posterior root of a spinal nerve. _spg._ Ganglion on posterior root of spinal nerve. _v.r._ Vertebral rudiment. _w._ White matter of spinal cord. _y._ Point where the spinal cord became segmented off from the superjacent epiblast.

Figs. 1, 2, and 3. Three sections of a Pristiurus embryo belonging to stage I. Fig. 1 passes through the heart, fig. 2 through the anterior part of the dorsal region, fig. 3 through a point slightly behind this. (Zeiss CC, ocul. 2.) In fig. 3 there is visible a slight proliferation of cells from the dorsal summit of the neural canal. In fig. 2 this proliferation definitely constitutes two club-shaped masses of cells (_pr_)--the rudiments of the posterior nerve-roots,--both attached to the dorsal summit of the spinal cord. In fig. 1 the rudiments of the posterior roots are of considerable length.

Fig. 4. Section through the dorsal region of a Torpedo embryo slightly older than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The section shews the formation of a pair of dorsal nerve-rudiments (_pr_) and a ventral nerve-rudiment (_ar_). The latter is shewn in its youngest condition, and is not distinctly cellular.

Fig. 5. Section through the dorsal region of a Torpedo embryo slightly younger than stage K. (Zeiss CC, ocul. 2.) The connective-tissue cells are omitted. The rudiment of the ganglion (_spg_) on the posterior root has appeared, and the junction of posterior root with the cord is difficult to detect. The anterior root forms an elongated cellular structure.

Fig. 6. Section through the dorsal region of a Pristiurus embryo of stage K. (Zeiss CC, ocul. 2.) The section especially illustrates the attachment of the posterior root to the spinal cord.

Fig. 7. Section through the same embryo as fig. 6. (Zeiss CC, ocul. 1.) The section contains an anterior root, which takes its origin at a point opposite the interval between two posterior roots.

Fig. 8. A series of posterior roots with their central ends united by a dorsal commissure, from a longitudinal and vertical section of a Scyllium embryo belonging to a stage intermediate between L and M. The embryo was hardened in a mixture of osmic and chromic acids.

Fig. 9. The central end of a posterior nerve-root from the same embryo, with the commissure springing out from it on either side.

EXPLANATION OF PLATES 15, 16, 17. (X. p. 397.)

PLATE 15. (THE HEAD DURING STAGES G-K.)

COMPLETE LIST OF REFERENCE LETTERS.

1_aa_, 2_aa_, etc. 1st, 2d, etc. aortic arch. _acv._ Anterior cardinal vein. _al._ Alimentary canal. _ao._ Aorta. _au._ Thickening of epiblast to form the auditory pit. _aun._ Auditory nerve. _aup._ Auditory pit. _auv._ Auditory vesicle. _b._ Wall of brain. _bb._ Base of brain. _cb._ Cerebellum. _cer._ Cerebrum. _Ch._ Choroid slit. _ch._ Notochord. _com._ Commissure connecting roots of vagus nerve. 1, 2, 3 etc. _eg._ External gills. _ep._ External epiblast. _fb._ Fore-brain. _gl._ Glossopharyngeal nerve. _hb._ Hind-brain. _ht._ Heart. _hy._ Hyaloid membrane. _In._ Infundibulum. _l._ Lens. _M._ Mouth involution. _m._ Mesoblast at the base of the brain. _mb._ Mid-brain. _mn._ v. Mandibular branch of fifth. _ol._ Olfactory pit. _op._ Eye. _opn._ Optic nerve. _opv._ Optic vesicle. _opth_V. Ophthalmic branch of fifth. _p._ Posterior root of spinal nerve. _pn._ Pineal gland. 1, 2 etc. _pp._ First, second, etc. section of body-cavity in the head. _pt._ Pituitary body. _so._ Somatopleure. _sp._ Splanchnopleure. _spc._ Spinal cord. _Th._ Thyroid body. _v._ Blood-vessel. iv._v._ Fourth ventricle. v. Fifth nerve. _Vc._ Visceral cleft. _Vg._ Vagus. vii. Seventh or facial nerve.

Fig. 1. Head of a Pristiurus embryo of stage K viewed as a transparent object.

The points which deserve special attention are: (1) The sections of the body-cavity in the head (_pp_): the first or premandibular section being situated close to the eye, the second in the mandibular arch. Above this one the fifth nerve bifurcates. The third at the summit of the hyoid arch.

The cranial nerves and the general appearance of the brain are well shewn in the figure.

The notochord cannot be traced in the living embryo so far forward as it is represented. It has been inserted according to the position which it is seen to occupy in sections.

Fig. 2. Head of an embryo of Scyllium canicula somewhat later than stage K, viewed as a transparent object.

The figure shews the condition of the brain; the branches of the fifth and seventh nerves (v. vii.); the rudiments of the semicircular canals; and the commencing appearance of the external gills as buds on both walls of 2nd, 3rd, and 4th clefts. The external gills have not appeared on the first cleft or spiracle.

Fig. 3. Section through the head of a Pristiurus embryo during stage G. It shews (1) the fifth nerve (v.) arising as an outgrowth from the dorsal summit of the brain. (2) The optic vesicles not yet constricted off from the fore-brain.

Figs. 4_a_ and 4_b_. Two sections through the head of a Pristiurus embryo of stage I. They shew (1) the appearance of the seventh nerve. (2) The portion of the body-cavity belonging to the first and second visceral arches. (3) The commencing thickening of epiblast to form the auditory involution.

In 4_b_, the posterior of the two sections, no trace of an auditory nerve is to be seen.

Figs. 5_a_ and 5_b_. Two sections through the head of a Torpedo embryo with 3 visceral clefts. Zeiss A, ocul. 1.

5_a_ shews the formation of the thin roof of the fourth ventricle by a divarication of the two lateral halves of the brain.

Both sections shew the commencing formation of the thyroid body (_th_) at the base of the mandibular arch.

They also illustrate the formation of the visceral clefts by an outgrowth from the alimentary tract without any corresponding ingrowth of the external epiblast.

Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo. Zeiss A, ocul. 1.

The section shews (1) the attachment of a branch of the vagus to the walls of the hind-brain. (2) The peculiar form of the hind-brain.

Fig. 7. Transverse section through the head of a Pristiurus embryo belonging to a stage intermediate between I and K, passing through both the fore-brain and the hind-brain. Zeiss A, ocul. 1.

The section illustrates (1) the formation of the pituitary body (_pt_) from the mouth involution (_m_), and proves that, although the wall of the throat (_al_) is in contact with the mouth involution, there is by this stage no communication between the two. (2) The eye. (3) The sections of the body-cavity in the head (1_pp_, 2_pp_). (4) The fifth nerve (v.) and the seventh nerve (vii).

Fig. 8. Transverse section through the brain of a rather older embryo than fig. 7. It shews the ventral junction of the anterior sections of the body-cavity in the head (1_pp_).

Figs. 9_a_ and 9_b_. Two longitudinal sections through the brain of a Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss A, ocul. 1.

9_a_ is taken through the median line, but is reconstructed from two sections. It shews (1) The divisions of the brain--The cerebrum and thalamencephalon in the fore-brain; the mid-brain; the commencing cerebellum in the hind-brain. (2) The relation of the mouth involution to the infundibulum. (3) The termination of the notochord.

9_b_ is a section to one side of the same brain. It shews (1) The divisions of the brain. (2) The point of outgrowth of the optic nerves (_opn_). (3) The sections of the body-cavity in the head and the bifurcation of the optic nerve over the second of these.

Fig. 10. Longitudinal section through the head of a Pristiurus embryo somewhat younger than fig. 9. Zeiss a, ocul. 4. It shews the relation of the nerves and the junction of the fifth, seventh, and auditory nerves with the brain.

Fig. 11. Longitudinal section through the fore-brain of a Pristiurus embryo of stage K, slightly to one side of the middle line. It shews the deep constriction separating the thalamencephalon from the cerebral hemispheres.

Fig. 12. Longitudinal section through the base of the brain of an embryo of a stage intermediate between I and K.

It shews (1) the condition of the end of the notochord; (2) the relation of the mouth involution to the infundibulum.

Fig. 13_a_. Longitudinal and horizontal section through part of the head of a Pristiurus embryo rather older than K. Zeiss A, ocul. 1.

The figure contains the eye cut through in the plane of the choroid slit. Thus the optic nerve (_opn_) and choroid slit (_ch_) are both exhibited. Through the latter is seen passing mesoblast accompanied by a blood-vessel (_v_). _Op_ represents part of the optic vesicle to one side of the choroid slit.

No mesoblast can be seen passing round the outside of the optic cup; and the only mesoblast which enters the optic cup passes through the choroid slit.

Fig. 13_b_. Transverse section through the last arch but one of the same embryo as 13_a_. Zeiss A, ocul. 1.

The figure shews (1) The mode of formation of a visceral cleft without any involution of the external skin. (2) The head-cavity in the arch and its situation in relation to the aortic arch.

Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 13, considerably magnified. The specimen was prepared by removing the nasal pit, flattening it out and mounting in glycerine after treatment with chromic acid. It shews the primitive arrangement of the Schneiderian folds. One side has been injured.

Figs. 15_a_ and 15_b_. Two longitudinal and vertical sections through the head of a Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3.

15_a_ is the most superficial section of the two. It shews the constitution of the seventh and fifth nerves, and of the intestinal branch of the vagus. The anterior branch of the seventh nerve deserves a special notice.

15_b_ mainly illustrates the dorsal commissure of the vagus nerve (_com_) continuous with the dorsal commissures of the posterior root of the spinal nerves.

Fig. 16. Two longitudinal and vertical sections of the head of a Pristiurus embryo belonging to the end of stage K. Zeiss a, ocul. 1.

16_a_ passes through the median line of the brain and shews the infundibulum, notochord and pituitary body, etc.

The pituitary body still opens into the mouth, though the septum between the mouth and the throat is broken through.

16_b_ is a more superficial section shewing the head-cavities _pp_ 1, 2, 3, and the lower vagus commissure.

PLATE 16. (X. p. 397.)

COMPLETE LIST OF REFERENCE LETTERS.

_auv._ Auditory vesicle. _cb._ Cerebellum. _cer._ Cerebral hemispheres. _ch._ Notochord. _cin._ Internal carotid. _ft._ Fasciculi teretes. _in._ Infundibulum. _lv._ Lateral ventricle. _mb._ Mid-brain, or optic lobes. _md._ Medulla oblongata. _mn._ Mandible. _ol._ Olfactory pit. _oll._ Olfactory lobe. _op._ Eye. _opn._ Optic nerve. _opth._ Optic thalamus. _pc._ Posterior commissure. _pcl._ Posterior clinoid. _pn._ Pineal gland. _pt._ Pituitary body. _rt._ Restiform tracts. _tv._ Tela vasculosa of the roof of the fourth ventricle. iv._v._ Fourth ventricle. vii. Seventh nerve. _x._ Rudiment of septum which will grow backwards and divide the unpaired cerebral rudiment into the two hemispheres.

Figs. 1_a_, 1_b_, 1_c_. Longitudinal sections of the brain of a Scyllium embryo belonging to stage L. Zeiss a, ocul. 1.

1_a_ is taken slightly to one side of the middle line, and shews the general features of the brain, and more especially the infundibulum (_in_) and pituitary body (_pt_).

1_b_ is through the median line of the pineal gland.

1_c_ is through the median line of the base of the brain, and shews the notochord (_ch_) and pituitary body (_pt_); the latter still communicating with the mouth. It also shews the wide opening of the infundibulum in the middle line into the base of the brain.

Fig. 2. Section through the unpaired cerebral rudiment during stage O, to shew the origin of the olfactory lobe and the olfactory nerve. The latter is seen to divide into numerous branches, one of which passes into each Schneiderian fold. At its origin are numerous ganglion cells represented by dots. Zeiss a, ocul. 2.

Fig. 3. Horizontal section through the three lobes of the brain during stage O. Zeiss a, ocul. 2.

The figure shews (1) the very slight indications which have appeared by this stage of an ingrowth to divide the cerebral rudiment into two lobes (_x_): (2) the optic thalami united by a posterior commissure, and on one side joining the base of the mid-brain, and behind them the pineal gland: (3) the thin posterior wall of the cerebral rudiment with folds projecting into the cerebral cavity.

Figs. 4_a_, 4_b_, 4_c_. Views from the side, from above, and from below, of a brain of Scyllium canicula during stage P. In the view from the side the eye (_op_) has not been removed.

The bilobed appearance both of the mid-brain and cerebellum should be noticed.

Fig. 5. Longitudinal section of a brain of Scyllium canicula during stage P. Zeiss a, ocul. 2.

There should be noticed (1) the increase in the flexure of the brain accompanying a rectification of the cranial axis; (2) the elongated pineal gland, and (3) the structure of the optic thalamus.

Figs. 6_a_, 6_b_, 6_c_. Views from the side, from above, and from below, of a brain of Scyllium stellare during a slightly later stage than Q.

Figs. 7_a_ and 7_b_. Two longitudinal sections through the brain of a Scyllium embryo during stage Q. Zeiss a, ocul. 2.

7_a_ cuts the hind part of the brain nearly through the middle line; while 7_b_ cuts the cerebral hemispheres and pineal gland through the middle.

In 7_a_ the infundibulum (1), cerebellum (2), the passage of the restiform tracts (_rt_) into the cerebellum (3), and the rudiments of the tela vasculosa (4) are shewn. In 7_b_ the septum between the two lobes of the cerebral hemispheres (1), the pineal gland (2), and the relations of the optic thalami (3) are shewn.

Figs. 8_a_, 8_b_, 8_c_, 8_d_. Four transverse sections of the brain of an embryo slightly older than Q. Zeiss a, ocul. 1.

8_a_ passes through the cerebral hemispheres at their junction with the olfactory lobes. On the right side is seen the olfactory nerve coming off from the olfactory lobe. At the dorsal side of the hemispheres is seen the pineal gland (_pn_).

8_b_ passes through the mid-brain now slightly bilobed, and the opening into the infundibulum (_in_). At the base of the section are seen the optic nerves and their chiasma.

8_c_ passes through the opening from the ventricle of the mid-brain into that of the cerebellum. Below the optic lobes is seen the infundibulum with the rudiments of the sacci vasculosi.

8_d_ passes through the front end of the medulla, and shews the roots of the seventh pair of nerves, and the overlapping of the medulla by the cerebellum.

PLATE 17. (X. p. 397.)

COMPLETE LIST OF REFERENCE LETTERS.

vii._a._ Anterior branch of seventh nerve. _ar._ Anterior root of spinal nerve. _auv._ Auditory vesicle. _cer._ Cerebrum. _ch._ Notochord. _ch._ Epithelial layer of choroid membrane. _gl._ Glossopharyngeal nerve. vii._hy._ Hyoid branch of seventh nerve. _hym._ Hyaloid membrane. _ll._ Lateral line. v._mn._ Ramus mandibularis of fifth nerve. vii._mn._ Mandibular (spiracular) branch of seventh nerve. v._mx._ Ramus maxillæ superioris of fifth nerve. _nl._ Nervus lateralis. _ol._ Olfactory pit. _op._ Eye. v._opth._ Ramus ophthalmicus of fifth nerve. _pch._ Parachordal cartilage. _pfal._ Processus falciformis. _pp._ Head cavity. _pr._ Posterior root of spinal nerve. _rt._ Retina. _sp._ Spiracle. v. Fifth nerve. vii. Seventh nerve. _vc._ Visceral cleft. _vg._ Vagus nerve. _vg.br._ Branchial branch of vagus. _vgcom._ Commissure uniting the roots of the vagus, and continuous with commissure uniting the posterior roots of the spinal nerves. _vgr._ Roots of vagus nerves in the brain. _vgin._ Intestinal branch of vagus. _vh._ Vitreous humour.

Fig. 1. Diagram of cranial nerves at stage L.

A description of the part of this referring to the vagus and glossopharyngeal nerves is given at p. 426. It should be noticed that there are only five strands indicated as springing from the spinal cord to form the vagus and glossopharyngeal nerves. It is however probable that there are even from the first a greater number of strands than this.

Fig. 2. Section through the hinder part of the medulla oblongata, stage between K and L. Zeiss A, ocul. 2.

It shews (1) the vagus commissure with branches on one side from the medulla: (2) the intestinal branch of the vagus giving off a nerve to the lateral line.

Fig. 3. Longitudinal and vertical section through the head of a Scyllium embryo of stage L. Zeiss a, ocul. 2.

It shews the course of the anterior branch of the seventh nerve (vii.); especially with relation to the ophthalmic branch of the fifth nerve (v._oth_).

Figs. 4_a_ and 4_b_. Two horizontal and longitudinal sections through the head of a Scyllium embryo belonging to stage O. Zeiss a, ocul. 2.

4_a_ is the most dorsal of the two sections, and shews the course of the anterior branch of the seventh nerve above the eye.

4_b_ is a slightly more ventral section, and shews the course of the fifth nerve.

Fig. 5. Longitudinal and horizontal section through the hind-brain at stage O, shewing the roots of the vagus and glossopharyngeal nerves in the brain. Zeiss B, ocul. 2.

There appears to be one root in the brain for the glossopharyngeal, and at least six for the vagus. The fibres from the roots divide in many cases into two bundles before leaving the brain. Swellings of the brain towards the interior of the fourth ventricle are in connection with the first five roots of the vagus, and the glossopharyngeal root; and a swelling is also intercalated between the first vagus root and the glossopharyngeal root.

Fig. 6. Horizontal section through a part of the choroid slit at stage P. Zeiss B, ocul. 2.

The figure shews (1) the rudimentary processus falciformis (_pfal_) giving origin to the vitreous humour; and (2) the hyaloid membrane (_hym_) which is seen to adhere to the retina, and not to the vitreous humour or processus falciformis.

EXPLANATION OF PLATE 18. (X. p. 446.)

COMPLETE LIST OF REFERENCE LETTERS.

_Nervous System._

_ar._ Anterior root of spinal nerve. _nc._ Neural canal. _pr._ Posterior root of spinal nerve. _spn._ Spinal nerve. _syg._ Sympathetic ganglion.

_Alimentary Canal._

_al._ Alimentary canal. _alv._ Caudal vesicle of the postanal gut. _clal._ Cloacal section of alimentary canal. _du._ Duodenum. _hpd._ Ductus choledochus. _pan._ pancreas. _soes._ Solid oesophagus. _spv._ Intestine with rudiment of spiral valve. _umc._ Umbilical canal.

_General._

_ao._ Dorsal aorta. _aur._ Auricle of heart. _cav._ Cardinal vein. _ch._ Notochord. _eppp._ Epithelial lining of the body-cavity. _ir._ Interrenal body. _me._ Mesentery. _mp._ Muscle-plate. _mpl_. Muscle-plate sending a prolongation into the limb. _po._ Primitive ovum. _pp._ Body-cavity. _sd._ Segmental duct. _st._ Segmental tube. _ts._ Tail swelling. _vcau._ Caudal vein. _x._ Subnotochordal rod.

Fig. 1. Transverse section through the anterior abdominal region of an embryo of a stage between K and L. Zeiss B, ocul. 2. Reduced one-third.

The section illustrates the junction of a sympathetic ganglion with a spinal nerve and the sprouting of the muscle-plates into the limbs (_mpl_).

Fig. 2. Transverse section through the abdominal region of an embryo belonging to stage L. Zeiss B, ocul. 2. Reduced one-third.

The section illustrates the junction of a sympathetic ganglion with a spinal nerve, and also the commencing formation of a branch from the aorta (still solid) which will pass through the sympathetic ganglion, and forms the first sign of the conversion of part of a sympathetic ganglion into one of the suprarenal bodies.

Fig. 3. Longitudinal and vertical section of an embryo of a stage between L and M, shewing the successive junctions of the spinal nerves and sympathetic ganglia.

Fig. 4. Section through the solid oesophagus during stage L. Zeiss A, ocul. 1. The section is taken through the region of the heart, so that the cavity of the auricle (_aur_) lies immediately below the oesophagus.

Fig. 5. Optical section of the tail of an embryo between stages I and K, shewing the junction between the neural and alimentary canals.

Fig. 6. Four sections through the caudal region of an embryo belonging to stage K, shewing the condition of the postanal section of the alimentary tract. Zeiss A, ocul. 2. An explanation of these figures is given on p. 449.

Fig. 7. Section through the interrenal body of a Scyllium embryo belonging to stage Q. Zeiss C, ocul. 2.

Fig. 8. Portion of a section of the interrenal body of an adult Scyllium. Zeiss C, ocul. 2.

EXPLANATION OF PLATE 19. (X. p. 460)

COMPLETE LIST OF REFERENCE LETTERS.

_Nervous System._

_n._ Nerve. _spn._ Spinal nerve. _syg._ Sympathetic ganglion.

_Alimentary Canal._

_cl._ Cloaca. _incl._ Cloacal involution. _oeep._ OEsophageal epithelium. _pan._ Pancreas. _th._ Thyroid body.

_General._

_abp._ Abdominal pocket (pore). _aur._ Auricle. _cav._ Cardinal vein. _cauv._ Caudal vein. _ly._ Lymphoid tissue. _mm._ Muscles. _od._ Oviduct. _pc._ Pericardium. _pp._ body-cavity. _sr._ Suprarenal body. _u._ Ureter. _vao._ Ventral aorta (anterior continuation of bulbus arteriosus). _ven._ Ventricle. _wd._ Wolffian duct.

Figs. 1_a_, 1_b_, 1_c_. Three sections through the cloacal region of an embryo belonging to stage O. 1_a_ is the anterior of the three sections. Zeiss A, ocul. 2. Reduced one-third.

1_a_ shews the cloacal involution at its deepest part abutting on the cloacal section of the alimentary tract.

1_b_ is a section through a point somewhat behind this close to the opening of the Wolffian ducts into the cloaca.

1_c_ shews the opening to the exterior in the posterior part of the cloaca, and also the rudiments of the two abdominal pockets (_abp_).

Fig. 2. Section through the cloacal region of an embryo belonging to stage P. Zeiss A, ocul. 2.

The figure shews the solid anterior extremity of the cloacal involution.

Fig. 3. Longitudinal vertical section through the thyroid body in a stage between O and P. Zeiss a a, ocul. 1.

The figure shews the solid thyroid body (_th_) connected in front with throat, and terminating below the bulbus arteriosus.

Fig. 4. Pancreas (_pan_) and adjoining part of the alimentary tract in longitudinal section, from an embryo between stages L and M. Zeiss A, ocul. 2.