Part 1

THE WORKS

OF

FRANCIS MAITLAND BALFOUR.

VOL. IV.

Memorial Edition.

Cambridge:

PRINTED BY C. J. CLAY, M.A. AND SON, AT THE UNIVERSITY PRESS.

Memorial Edition.

THE WORKS

OF

FRANCIS MAITLAND BALFOUR,

M.A., LL.D., F.R.S.,

FELLOW OF TRINITY COLLEGE, AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF CAMBRIDGE.

EDITED BY

M. FOSTER, F.R.S., PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE;

AND

ADAM SEDGWICK, M.A., FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE.

VOL. IV.

PLATES.

London: MACMILLAN AND CO. 1885

[_The Right of Translation is reserved._]

LIST OF PLATES, ILLUSTRATING THE ORIGINAL MEMOIRS IN VOL. I.

PAGE

Plate 1. Development of the layers of the blastoderm 29

" Disappearance of primitive groove 41

" 2. Development of blood-vessels 47

" 3.} Preliminary account of development of Elasmobranch " 4.} Fishes 60

" 5. Comparison of early stages of Vertebrates 112

" 6. Development of Elasmobranch Fishes. ch. II. 222

" 7. " " " " III. 246

" 8.} " 9.} " " " " IV. 286

" 10. " " " " V. 298 " 11.} " 12.} " " " " VI. 315

" 13. " " " " VII. 361

" 14. " " " " VIII. 378

" 15.} " 16.} " " " " IX. 397 " 17.}

" 18. " " " " X. 446

" 19. " " " " XI. 460

" 20.} " 21.} " " " " XII. 479

" 22.} " 23.} Development of spinal nerves in Elasmobranchs 168

" 24.} " 25.} Structure and development of Vertebrate ovary 549 " 26.}

" 27.} " 28.} Head-kidney in embryo Chick 618

" 29. Early development of Lacertilia 644

" 30.} " 31.} Development of Araneina 668 " 32.}

" 33. Development of paired fins of Elasmobranchii 714

" 34.} " 35.} " 36.} " 37.} " 38.} Structure and development of Lepidosteus 738 " 39.} " 40.} " 41.} " 42.}

" 43.} " 44.} Germinal layers of the Chick 854 " 45.}

" 46.} " 47.} " 48.} " 49.} Anatomy and development of Peripatus Capensis 871 " 50.} " 51.} " 52.} " 53.}

EXPLANATION OF PLATES.

EXPLANATION OF PLATE 1. Figs. 1-5 and 9-12. (II. p. 29.)

Fig. 1. Section through an unincubated blastoderm, shewing the upper layer, composed of a single row of columnar cells, and the lower layer, composed of several rows of rounded cells in which no nucleus is visible. Some of the "formative cells," at the bottom of the segmentation cavity, are seen at (_b_).

Fig. 2. Section through the periphery of an eight hours' blastoderm, shewing the epiblast (_p_), the hypoblast (_h_), and the mesoblast commencing to be formed (_c_), partly by lower-layer cells enclosed between the epiblast and hypoblast, and partly by formative cells. Formative cells at the bottom of the segmentation cavity are seen at _b_. At _s_ is one of the side folds parallel to the primitive groove.

Fig. 3. Portion of the hypoblast of a thirteen hours' blastoderm, treated with silver nitrate, shewing the great variation in the size of the cells at this period. An hour-glass shaped nucleus is seen at _a_.

Fig. 4. Periphery of a twenty-three hours' blastoderm, shewing cell for cell the junction between the hypoblast (_h_) and white-yolk spheres (_w_).

Fig. 5. Junction between the white-yolk spheres and the hypoblast cells at the passage from the area pellucida to the area opaca. The specimen was treated with silver nitrate to bring out the shape of the cells. The line of junction between the opaque and pellucid areas passes diagonally.

Fig. 9. Section through the primitive streak of an eight hours' blastoderm. The specimen shews the mesoblast very much thickened in the immediate neighbourhood of the primitive streak, but hardly formed at all on each side of the streak. It also shews the primitive groove just beginning to be formed (_pr_), and the fusion between the epiblast and the mesoblast under the primitive groove. The hypoblast is completely formed in the central part of the blastoderm. At _f_ is seen one of the side folds parallel to the primitive groove. Its depth has been increased by the action of the chromic acid.

Fig. 10. Hypoblast cells from the hinder end of a thirty-six hours' embryo, treated with silver nitrate, shewing the regularity and elongated shape of the cells over the embryo and the smaller cells on each side.

Fig. 11. Epiblast cells from an unincubated blastoderm, treated with silver nitrate, shewing the regular hexagonal shape of the cells and the small spherules they contain.

Fig. 12. Portion of the epiblast of a thirty-six hours' embryo, treated with silver nitrate, shewing the small rounded cells frequently found at the meeting-points of several larger cells which are characteristic of the upper layer.

EXPLANATION OF PLATE 1. Figs. 6-8 and 13-19. (III. p. 41.)

Figs. 6 and 7 are sections through an embryo rather earlier than the one drawn in fig. 8. Fig. 6 passes through the just commencing medullary groove (_md_), which appears in fresh specimens, as in fig. 8, merely as an opaque streak coming from the end of the primitive groove. The notochord is hardly differentiated, but the _complete_ separation of mesoblast and hypoblast under the primitive groove is clearly shewn. Fig. 7 passes through the anterior end of the primitive groove (_pr_), and shews the fusion between the mesoblast and epiblast, which is always to be found under the primitive groove.

Fig. 8 is a view from above of a twenty hours' blastoderm, seen as a transparent object. Primitive groove (_pr_). Medullary groove (_md_), which passes off from the anterior end of the primitive groove, and is produced by the thickening of the mesoblast. Head fold (_pf_).

Figs. 13-17 are sections through the blastoderm, drawn in fig. 18 through the lines 1, 2, 3, 4, 5 respectively.

The first section (fig. 13) passes through the true medullary groove (_mc_); the two medullary folds (_A_, _A_) are seen on each side with the thickened mesoblast, and the mesoblast cells are beginning to form the notochord (_nc_) under the medullary groove. There is no adherence between the mesoblast cells and the epiblast under the medullary groove.

The second (fig. 14) section passes through the medullary groove where it has become wider. Medullary folds, _A_, _A_; notochord, _ch_.

In the third section (fig. 15) the notochord (_ch_) is broader, and the epiblast is raised in the centre, while the medullary folds are seen far apart at _A_.

In section fig. 16 the medullary folds (_A_) are still to be seen enclosing the anterior end of the primitive groove (_pr_). Where the primitive groove appears there is a fusion of the epiblast and mesoblast, and no appearance of the notochord.

In the last section, fig. 17, no trace is to be seen of the medullary folds.

Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is twenty-three hours old; fig. 19 twenty-five hours. They both shew how the medullary canal arises entirely independently of the primitive groove and in front of it, and also how the primitive groove gets pushed backwards by the growth of the medullary groove. _pv_, Protovertebræ; other references as above. Fig. 18 is the blastoderm from which sections figs. 13-17 were cut.

EXPLANATION OF PLATE 2. (IV. p. 47.)

Fig. 1 is taken from the anterior part of the pellucid area of a thirty hours' chick, with four protovertebræ. At _n_ is a nucleus with two nucleoli.

Figs. 2 and 3 are taken from the posterior end of the pellucid area of a chick with eight protovertebræ. In fig. 3 the nuclei are seen to have considerably increased in number at the points of starting of the protoplasmic processes. At _n_ is seen a nucleus with two nucleoli.

Fig. 4 is taken from the anterior part of the pellucid area of an embryo of thirty-six hours. It shews the narrow processes characteristic of the anterior part of the pellucid area, and the fewer nuclei. Small spaces, which have the appearance of vacuoles, are shewn at _v_.

Fig. 5 is taken from the posterior part of the pellucid area of a thirty-six hours' embryo. It shews the nuclei, with somewhat irregular nucleoli, which have begun to acquire the red colour of blood-corpuscles; the protoplasmic processes containing the nuclei; the nuclei in the protoplasm surrounding the corpuscles, as shewn at _a_, _a´_.

Fig. 6 shews fully formed blood-vessels, in part filled with blood-corpuscles and in part empty. The walls of the capillaries, formed of cells, spindle-shaped in section, are shewn, and also the secondary investment of Klein at _k_, and at _b_ is seen a narrow protoplasmic process filled with blood-corpuscles.

Fig. 7 is taken from the anterior part of the pellucid area of a thirty-six hours' embryo. It shews a collection of nuclei which are beginning to become blood-corpuscles.

Figs. 1-5 are drawn with an 1/8 object-glass. Fig. 6 is on a much smaller scale. Fig. 7 is intermediate.

Fig. 8. A transverse section through the dorsal region of a forty-five hours' embryo; _ao_, aorta with a few blood-corpuscles. v, Blood-vessels, all of them being formed in the splanchnopleure, and all of them provided with the secondary investment of Klein; _pe_, pellucid area; _op_, opaque area.

Fig. 9. Small portion of a section through the opaque area of a thirty-five hours' embryo, showing protoplasmic processes, with nuclei passing from the somatopleure to the splanchnopleure.

Fig. 10. Section through the heart of a thirty-four hours' embryo. _a_. Alimentary canal; _hb_, hind brain; _nc_, notochord; _e_, epiblast; _so_, mesoblast of the somatopleure; _sp_, mesoblast of the splanchnopleure; _hy_, hypoblast; _hz_, cavity of the heart.

Fig. 11. Section through the same embryo as fig. 10, and passing through the orifice of the omphalomeseraic vein. _of_, Omphalomeseraic vein; other references as above.

These two sections shew that the heart is entirely formed from the mesoblast of the _splanchnopleure_, and that it is formed by the splitting of that part of the mesoblast which has turned to assume its normal direction after being folded in to form the muscular wall of the alimentary canal. In fig. 11 the cavities so formed on each side have not yet united, but in fig. 10 they have united. When the folding becomes more complete the cavities (_of_, _of_) in fig. 11 will unite, and in this way the origin of the omphalomeseraic veins will be carried further backwards. In the section immediately behind section 11 the mesoblast had become thickened, but had not split.

EXPLANATION OF PLATES 3 AND 4. (V. p. 60.)

COMPLETE LIST OF REFERENCE LETTERS.

_al._ Alimentary canal. _ao._ Dorsal aorta. _auv._ Auditory vesicle. _bd._ Formative cell probably derived from the yolk. _cav._ Cardinal vein. _ch._ Notochord. _ch´._ Thickening of hypoblast to form the notochord. _eb._ Line indicating the edge of the blastoderm. _ep._ Epiblast. _ep´._ Epidermis. _er._ Embryonic rim. _es._ Embryonic swelling. _gl._ Glosso-pharyngeal nerve. _h._ Head. _ht._ Heart. _hy._ Hypoblast. _ll._ Lower layer cells. _ly._ Line of separation between the blastoderm and the yolk. _m._ Mesoblast. _mc._ Medullary canal. _mg._ Medullary groove. _mp._ Muscle-plate. _mp´._ Early formed mass of muscles. _n._ Peculiar nuclei formed in the yolk. _n´._ Similar nuclei in the cells of the blastoderm. _na._ Cells which help to close in the alimentary canal, and which are derived from the yolk. _ny._ Network of lines present in the food-yolk. _ol._ Olfactory pit. _op._ Eye. _ov._ Oviduct. _pn._ Pineal gland. _pov._ Projection which becomes the ovary. _pp._ Pleuro-peritoneal cavity. _pp´._ Remains of pleuro-peritoneal cavity in the head. _prv._ Protovertebræ. _pwd._ Primary points of involution from the pleuro-peritoneal cavity by the coalescence of which the Wolffian duct is formed. _sg._ Segmentation cavity. _so._ Somatopleure. _sos._ Stalk connecting embryo with yolk-sac. _sp._ Splanchnopleure. _spn._ Spinal nerve. _sur._ Suprarenal body. _ts._ Caudal lobes. _v._ Blood-vessel. _vg._ Vagus nerve. V. Fifth nerve. VII. Seventh nerve. _vc_, 1, 2, 3, &c. 1st, 2nd and 3rd &c. visceral clefts. _vp._ Vertebral plates. _wd._ Wolffian duct. _x._ Peculiar body underlying the notochord derived from the hypoblast. _yk._ Yolk spherules.

All the figures were drawn with the Camera Lucida.

PLATE 3.

Fig. 1. Section parallel with the long axis of the embryo through a blastoderm, in which the floor of the segmentation cavity (_sg_) is not yet completely lined by cells. The roof of the segmentation cavity is broken. (Magnified 60 diam.) The section is intended chiefly to illustrate the distribution of nuclei (_n_) in the yolk under the blastoderm. One of the chief points to be noticed in their distribution is the fact that they form almost a complete layer under the floor of the segmentation cavity. This probably indicates that the cells whose nuclei they become take some share in forming the layer of cells which subsequently (vide fig. 4) forms the floor of the cavity.

Fig. 2. Small portion of blastoderm and subjacent yolk of an embryo at the time of the first appearance of the medullary groove. (Magnified 300 diam.)

The specimen is taken from a portion of the blastoderm which will form part of the embryo. It shews two large nuclei of the yolk (_n_) and the network in the yolk between them; this network is seen to be closer around the nuclei than in the intervening space. The specimen further shews that there are no areas representing cells around the nuclei.

Fig. 3. Section parallel with the long axis of the embryo through a blastoderm, in which the floor of the segmentation cavity is not yet covered by a complete layer of cells. (Magnified 60 diam.)

It illustrates (1) the characters of the epiblast, (2) the embryonic swelling (_es_), (3) the segmentation cavity (_sg_). It should have been drawn upon the same scale as fig. 4; the line above it represents its true length upon this scale.

Fig. 4. Longitudinal section through a blastoderm at the time of the first appearance of the embryonic rim, and before the formation of the medullary groove. (Magnified 45 diam.)

It illustrates (1) the embryonic rim, (2) the continuity of epiblast and hypoblast at edge of this, (3) the continual differentiation of the lower layer cells, to form, on the one hand, the hypoblast, which is continuous with the epiblast, and on the other the mesoblast, between this and the epiblast; (4) the segmentation cavity, whose floor of cells is now completed.

N.B. The cells at the embryonic end of the blastoderm have been made rather too large.

Fig. 5. Surface view of the blastoderm shortly after the appearance of the medullary groove. To shew the relation of the embryo to the blastoderm.

Fig. 6_a_ and _b_. Two transverse sections of the same embryo, shortly after the appearance of the medullary groove. (Magnified 96 diam.)

_a._ In the region of the groove. It shews (1) the two masses of mesoblast on each side, and the deficiency of the mesoblast underneath the medullary groove; (2) the commencement of the closing in of the alimentary canal below, chiefly from cells (_na_) derived from the yolk.

_b._ Section in the region of the head where the medullary groove is deficient, other points as above.

Fig. 7_a_ and _b_. Two transverse sections of an embryo about the age or rather younger than that represented in fig. 5. (Magnified 96 diam.)

_a._ Section nearer the tail; it shews the thickening of the hypoblast to form the notochord (_ch´_).

In _b_ the thickening has become completely separated from the hypoblast as the notochord. In _a_ the epiblast and hypoblast are continuous at the edge of the section, owing to the section passing through the embryonic rim.

Fig. 8. Surface view of a spatula-shaped embryo. The figure shews (1) the flattened head (_h_) where the medullary groove is deficient, (2) the caudal lobes, with a groove between them; it also shews that at this point, the medullary groove has become roofed over and converted into a canal.

Fig. 8_a_. Transverse section of fig. 8, passing through the line _a_. (Magnified 90 diam.) The section shews (1) the absence of the medullary groove in the head and the medullary folds turning down at this time instead of upwards; (2) the presence of the pleuro-peritoneal cavity in the head (_pp_); (3) the completely closed alimentary canal (_al_).

Fig. 8_b_. Transverse section of fig. 8, through the line _b_. (Magnified 90 diam.) It shews (1) the neural canal completely formed; (2) the vertebral plates of mesoblast not yet split up into somatopleure and splanchnopleure.

Fig. 9. Side view of an embryo of the Torpedo, seen as a transparent object a little older than the embryo represented in fig. 8. (Magnified 20 diam.) The internal anatomy has hardly altered, with the exception of the medullary folds having closed over above the head and the whole embryo having become more folded off from the germ.

The two caudal lobes, and the very marked groove between them, are seen at _ts_. The front end of the notochord became indistinct, and I could not see its exact termination. The epithelium of the alimentary canal (_al_) is seen closely underlying the notochord and becoming continuous with the epiblast at the hind end of the notochord.

The first visceral cleft (1_vc_) and eye (_op_) are just commencing to be formed, and the cranial flexure has just appeared.

Fig. 10. Section through the dorsal region of an embryo somewhat older than the one represented in fig. 9. (Magnified 96 diam.)

It shews (1) the formation by a pinching off from the top of the alimentary canal of a peculiar body which underlies the notochord (_x_); (2) the primitive extension of the pleuro-peritoneal cavity up to the top of the vertebral plates.

PLATE 4.

Fig. 11_a_, _b_, and _c_. Three sections closely following each other from an embryo in which three visceral clefts are present; _a_ is the most anterior of the three. (Magnified 96 diam.) In all of these the muscle-plates are shewn at _mp_. They have become separated from the lateral plates in _b_ and _c_, but are still continuous with them in _a_. The early formed mass of muscles is also shewn in all the figures (_mp´_).

The figures further shew (1) the formation of the spinal nerves (_spn_) as small bodies of cells closely applied to the upper and outer edge of the neural canal.

(2) The commencing formation of the cells which form the axial skeleton from the inner (splanchnopleuric) layer of the muscle-plate. Sections _b_ and _c_ are given more especially to shew the mode of formation of the oviduct (_ov_).

In _b_ it is seen as a _solid knob (ov)_, arising from the point where the somatopleure and splanchnopleure unite, and in _c_ (the section behind _b_) as a _solid rod (ov)_ closely applied to the epiblast, which has grown backwards from the knob seen in _b_.

N.B. In all three sections only one side is completed.

Fig. 12_a_ and _b_. Two transverse sections of an embryo just before the appearance of the external gills. (Magnified 96 diam.)

In _a_ there is seen to be an involution on each side (_pwd_), while _b_ is a section from the space between two involutions from the pleuro-peritoneal cavity, so that the Wolffian duct (at first solid) (_wd_) is not connected as in _a_ with the pleuro-peritoneal cavity. The further points shewn in the sections are--

(1) The commencing formation of the spiral valve (_al_). (2) The suprarenal body (_sur_). (3) The oviduct (_ov_), which has acquired a lumen. (4) The increase in length of the muscle-plates, the spinal nerves, &c.

Fig. 13. Section through the dorsal region of an embryo in which the external gills are of considerable length. (Magnified 40 diam.) The chief points to be noticed:

(1) The formation of the Wolffian body by outgrowths from the Wolffian duct (_wd_). (2) One of the still continuing connections (primitive involutions) between the Wolffian duct and the pleuro-peritoneal cavity (_pwd_). (3) The oviduct largely increased in size (_ov_). N.B. On the left side the oviduct has been accidentally made too small. (4) The growth downwards of the muscle-plate to form the muscles of the abdomen. (5) The formation of an outgrowth on each side of the mesentery (_pov_), which will become the ovary. (6) The spiral valve (_al_).

Fig. 14. Transparent view of the head of an embryo shortly before the appearance of the external gills. (Magnified 20 diam.) The chief points to be noticed are--

(1) The relation of the cranial nerves to the visceral clefts and the manner in which the glosso-pharyngeal (_gl_) and vagus (_vg_) are united. (2) The remnants of the pleuro-peritoneal cavity in the head (_pp_). (3) The eye (_op_). The stalk, as well as the bulb of the eye, are supposed to be in focus, so that the whole eye has a somewhat peculiar appearance.

EXPLANATION OF PLATE 5. (VI. p. 112.)

COMPLETE LIST OF REFERENCE LETTERS.

_al._ Cavity of alimentary canal. _bl._ Blastoderm. _ch._ Notochord. _ep._ Epiblast. _em._ Embryo. _f._ Formative cells. _hy._ Hypoblast. _ll._ Lower layer cells. _m._ Mesoblast. _n._ Nuclei of yolk of Selachian egg. _nc._ Neural canal. _sg._ Segmentation cavity. _x._ Point where epiblast and hypoblast are continuous at the mouth of the alimentary involution. This point is always situated at the tail end of the embryo. _yk._ Yolk.

Epiblast is coloured blue, mesoblast red, and hypoblast yellow. The lower layer cells before their separation into hypoblast and mesoblast are also coloured green.

A I, A II, A III. Diagrammatic sections of Amphioxus in its early stages (founded upon Kowalevsky's observations).

B I, B II, B III. Diagrammatic longitudinal sections of an hypothetical animal, intermediate between Amphioxus and Batrachians, in its early stages.

C I, C II, C III. Diagrammatic longitudinal sections of Bombinator igneus in its early stages (founded upon Götte's observations). In C III the neural canal is completed, which was not the case in B III. The epiblast in C III has been diagrammatically represented as a single layer.

D I, D II, D III. Diagrammatic longitudinal sections of an animal, intermediate between Batrachians and Selachians, in its early stages.

E I, E II, E III. Diagrammatic longitudinal sections of a Selachian in its early stages.

E´. Surface view of the yolk of a Selachian's egg to shew the manner in which it is enclosed by the Blastoderm. The yolk is represented yellow and the Blastoderm blue.

F I, F II, F III. Diagrammatic longitudinal sections of a Bird in its early

EXPLANATION OF PLATE 6. (X. p. 222.)

Fig. 1. Section through the germinal disc of a ripe ovarian ovum of the Skate. _gv._ germinal vesicle.

Fig. 2. Surface-view of a germinal disc with two furrows.

Figs. 3, 4, 5. Surface-views of three germinal discs in different stages of segmentation.

Fig. 6. Section through the germinal disc represented in fig 3. _n._ nucleus; _x._ edge of germinal disc. The engraver has not accurately copied my original drawings in respect to the structure of the segmentation furrows.

Figs. 6_a_ and 6_b_. Two furrows of the same germinal disc more highly magnified.

Fig. 6_c_. A nucleus from the same germinal disc highly magnified.