PART I.

SYSTEMATIC EMBRYOLOGY.

INTRODUCTION.

In all the Metazoa the segmentation is followed by a series of changes which result in the grouping of the embryonic cells into definite layers, or membranes, known as the germinal layers. There are always two of these layers, known as the epiblast and hypoblast; and in the majority of instances a third layer, known as the mesoblast, becomes interposed between them. It is by the further differentiation of the germinal layers that the organs of the adult become built up. Owing to this it is usual, in the language of Embryology, to speak of the organs as derived from such or such a germinal layer.

At the close of the section of this work devoted to systematic embryology, there is a discussion of the difficult questions which arise as to the complete or partial homology of these layers throughout the Metazoa, and as to the meaning to be attached to the various processes by which they take their origin; but a few words as to the general fate of the layers, and the general nature of the processes by which they are formed, will not be out of place here.

Of the three layers the epiblast and hypoblast are to be regarded as the primary. The epiblast is essentially the primitive integument, and constitutes the protective and sensory layer. It gives rise to the skin, cuticle, nervous system, and organs of special sense. The hypoblast is essentially the digestive and secretory layer, and gives rise to the epithelium lining the alimentary tract and the glands connected with it.

The mesoblast is only found in a fully developed condition in the forms more highly organized than the Coelenterata. It gives origin to the general connective tissue, internal skeleton, the muscular system, the lining of the body cavity, the vascular, and excretory systems. It probably in the first instance originated from differentiations of the two primary layers, and in all groups with a well-developed body cavity it is divided into two strata. One of them forms part of the body wall and is known as the somatic mesoblast, the other forms part of the wall of the viscera and is known as the splanchnic mesoblast.

A very large number not to say the majority of organs are derived from parts of two of the germinal layers. Many glands for instance have a lining of hypoblast which is coated by a mesoblastic layer.

[FIG. 55. DIAGRAM OF A GASTRULA. (From Gegenbaur.)

_a._ blastopore; _b._ archenteron; _c._ hypoblast; _d._ epiblast.]

The processes by which the germinal layers take their origin are largely influenced by the character of the segmentation, which, as was shewn in the last chapter, is mainly dependent on the distribution of the food-yolk. When the segmentation is regular, and results in the formation of a blastosphere, the epiblast and hypoblast are usually differentiated from the uniform cells forming the wall of the blastosphere in one of the two following ways.

(1) One-half of the blastosphere may be pushed in towards the other half. A two-layered hemisphere is thus established which soon elongates, while its opening narrows to a small pore (fig. 55). The embryonic form produced by this process is known as a gastrula. The process by which it originates is known as embolic invagination, or shortly invagination. Of the two layers of which it is formed the inner one (_c_) is known as the hypoblast and the outer (_d_) as the epiblast, while the pore leading into its cavity lined by the hypoblast is the blastopore (_a_). The cavity itself is the archenteron (_b_).

(2) The cells of the blastosphere may divide themselves by a process of concentric splitting into two layers (fig. 56, 3). The two layers are as before the epiblast and hypoblast, and the process by which they originate is known as delamination. The central cavity or archenteron (_F_) is in the case of delamination the original segmentation cavity; and not an entirely new cavity as in the case of invagination. By the perforation of the closed two-walled vesicle resulting from delamination an embryonic form is produced which cannot be distinguished in structure from the gastrula produced by invagination (fig. 56, 4). The opening (_M_) in this case is not however known as the blastopore but as the mouth.

[FIG. 56. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY DELAMINATION. (From Lankester.)

Fig. 1. Ovum. Fig. 2. Stage in segmentation. Fig. 3. Commencement of delamination after the appearance of a central cavity. Fig. 4. Delamination completed, mouth forming at _M_. In fig. 1, 2 and 3 _Ec._ is ectoplasm, and _En._ is entoplasm. In fig. 4 _Ec._ is epiblast and _En._ hypoblast.]

When segmentation does not take place on the regular type the processes above described are as a rule somewhat modified. The yolk is usually concentrated in the cells which would, in the case of a simple gastrula, be invaginated. As a consequence of this, these cells become (1) distinctly marked off from the epiblast cells during the segmentation; and (2) very much more bulky than the epiblast cells. The bulkiness of the hypoblast cells necessitates a modification of the normal process of embolic invagination, and causes another process to be substituted for it, viz. the growth of the epiblast cells as a thin layer over the hypoblast. This process (fig. 57) is known as epibolic invagination. The point where the complete enclosure of the hypoblast cells is effected is known as the blastopore. All intermediate conditions between epibolic and embolic invagination have been found.

[FIG. 57. TRANSVERSE SECTION THROUGH THE OVUM OF EUAXES DURING AN EARLY STAGE OF DEVELOPMENT. (After Kowalevsky.)

_ep._ epiblast; _ms._ mesoblastic band; _hy._ hypoblast.]

[FIG. 58. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM. (After Metschnikoff.)

A. Stage after the delamination. _ep._ epiblastic invagination to form pneumatocyst.

B. Later stage after the formation of the gastric cavity in the solid hypoblast. _po._ polypite; _t._ tentacle; _pp._ pneumatophore; _ep._ epiblastic invagination to form pneumatocyst; _hy._ hypoblast surrounding pneumatocyst.]

In delamination, when the segmentation is not uniform, or when a solid morula is formed, the differentiation of the epiblast and hypoblast is effected by the separation of the central solid mass of cells from the peripheral cells (fig. 58 A).

In the case of epibolic invagination as well as in that of the type of delamination just spoken of, the archenteric cavity is in most cases secondarily formed in the solid mass of hypoblast (fig. 58 B).

In ova with a partial segmentation there is usually some modification of the epibolic gastrula.

Many varieties are found in the animal kingdom of the types of invagination and delamination just characterized, and in not a few forms the layers originate in a manner which cannot be brought into connection with either of these processes.

[FIG. 59. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)

A. Stage when the four hypoblast cells are nearly enclosed. B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore. _ep._ epiblast; _me._ mesoblast; _bl._ blastopore.]

The mesoblast usually originates subsequently to the two primary layers. It then springs from one or both of the other layers, but its modes of origin are so various that it would be useless to attempt to classify them here. In cases of invagination it often arises at the lips of the blastopore (fig. 57 and 59), and in other cases part of it springs as paired hollow outgrowths of the walls of the archenteron. Such outgrowths are shewn in fig. 60, B and C at _pv_. The cavity of the outgrowths forms the body cavity, and the walls of the outgrowths the somatic and splanchnic layers of mesoblast (fig. C. _sp._ and _so._). The archenteron is in part always converted into a section of the permanent alimentary tract and the section of the alimentary tract so derived is known as the mesenteron. There are however usually two additional parts of the alimentary tract, known as the stomodaeum and proctodaeum, derived from epiblastic invaginations. They give rise respectively to the oral and anal extremities of the alimentary tract.

[FIG. 60. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after Bütschli and B after Kowalevsky.) The three embryos are represented in the same positions.

A. Represents the gastrula stage. B. Represents a succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the mesoblast. C. Represents a later stage in which the mouth involution (_m_) has become continuous with alimentary tract, and the blastopore has become closed.

_m._ mouth; _al._ alimentary canal; _ae._ archenteron; _bl._ _p._ blastopore; _pv._ perivisceral cavity; _sp._ splanchnic mesoblast; _so._ somatic mesoblast; _ge._ generative organs.]

BIBLIOGRAPHY.

(107) K. E. von Baer. "Ueb. Entwicklungsgeschichte d. Thiere." Königsberg, 1828-1837.

(108) C. Claus. _Grundzüge d. Zoologie._ Marburg und Leipzig, 1879.

(109) C. Gegenbaur. _Grundriss d. vergleichenden Anatomie._ Leipzig, 1878. _Vide_ also Translation. _Elements of Comparative Anatomy._ Macmillan and Co., 1878.

(110) E. Haeckel. _Studien z. Gastræa-Theorie_. Jena, 1877, and also _Jenaische Zeitschrift_, Vols. VIII. and IX.

(111) E. Haeckel. _Schöpfungsgeschichte._ Leipzig. _Vide_ also Translation. _The History of Creation._ King and Co., London, 1876.

(112) E. Haeckel. _Anthropogenie._ Leipzig. _Vide_ also Translation. _Anthropogeny_ (Translation). Kegan Paul and Co., London, 1878.

(113) Th. H. Huxley. _The Anatomy of Invertebrated Animals._ Churchill, 1877.

(114) E. R. Lankester. "Notes on Embryology and Classification." _Quart. J. of. Micr. Science_, Vol. XVII. 1877.

(115) A. S. P. Packard. _Life Histories of Animals, including Man, or Outlines of Comparative Embryology._ Holt and Co., New York, 1876.

(116) H. Rathke. _Abhandlungen z. Bildung und Entwicklungsgesch. d. Menschen u. d. Thiere._ Leipzig, 1833.