FORMATION OF THE EMBRYO 1125 



two complete nucleated cells make their appearance. These divide in 

 turn, till at length (in the mammal) the embryo is represented by a 

 hollow sphere or vesicle, with a cellular crust. During division the 

 upper or outer cells have always been larger than the inner and lower, 

 and have multiplied more rapidly; and thus it comes about that the 

 hollow sphere of large cells encloses a mass of smaller cells, along with 

 remnants of broken-down yolk and of fluid derived by absorption from 

 the contents of the uterus. The smaller cells continue to multiply and 

 arrange themselves as a lining to the sphere already formed, so that in a 

 short time it becomes double, and we have already differentiated two of 

 the primary embryonic layers the ectoderm, also called the epiblast, or 

 superficial, and the endoderm, also called the hypoblast, or deep layer. 

 The whole sphere is called the blastoderm, or the blastodermic vesicle. 

 While this inner shell of endodermic cells is gradually creeping on to 

 completion, there appears at a part where it is already fully formed a 

 small opaque whitish disc, the germinal area or embryonal shield. This 

 represents the stocks on which the framework of the embryo is to be laid 

 down. The area elongates; at its posterior end appears a thickened 

 line, the primitive streak, soon furrowed by a longitudinal groove, 'the 

 primitive groove, that marks the direction in which the long axis of the 

 future embryo will lie, but is not itself a permanent line in the building, 

 and ultimately vanishes. The appearance of the primitive streak is the 

 signal that a rapid proliferation of the cells of the germinal area, and 

 especially of the ectoderm, has begun ; and this goes on until a third layer 

 is formed, intermediate in position to the original two, and therefore 

 named the mesoderm. While this is pushing its way over the germinal 

 area and into the rest of the blastodermic vesicle, the ectoderm in front 

 of the primitive streak rises up in two lateral ridges, enclosing between 

 them the medullary groove. The medullary groove is the beginning of 

 the cerebro-spinal axis ; its walls first come to overhang the furrow, and 

 then to coalesce; and the medullary groove has now become the neural 

 canal. Immediately under it the mesoderm forms a rod of cells, the 

 notochord, which is the forerunner of the vertebral column ; around this 

 the bodies of the vertebrae are afterwards developed from cubical masses 

 of mesodermic cells, arranged in pairs along the notochord, and called 

 the protovertebra. The rest of the mesoderm, running out on each side 

 from the protovertebrae, splits into two layers, an upper or somatic layer, 

 which unites with the ectoderm, forming with it the somatopleure, and 

 a lower or splanchnic layer, which unites with the endoderm to form 

 the splanchnopleure. Between the somatopleure and the splanchno- 



fleure is a space called the coelom, or pleuro-peritoneal cavity (Fig. 485). 

 he layer of ectoderm which envelops the whole (termed the tropho- 

 blast, from its nutritive function), in conjunction with the underlying 

 mesoderm, represents the prechorion, the early stage of the chorion. 



Up to the present, apart from the enclosure of the neural canal, all this 

 formative activitv is buried beneath the surface of the blastoderm, and 

 has not showed itself by any external token ; the embryo still appears as 

 a portion of the germinal area, and lies in its plane. But now a pocket, 

 or crease, or moat, beginning at the head as the head-fold, then pushing 

 under the tail, gradually creeps round and undermines the whole 

 embryo, which is raised above the general level, and, as it were, scooped 

 out from the rest of the blastoderm; till at length it lies on the latter, 

 something like an upturned canoe, enclosing a tube, complete in front 

 and behind, but still open in the middle, where it communicates with 

 the interior of the yolk- vesicle. Since this tube has been formed by the 

 tucking in of the three ancestral layers of the blastoderm, it follows that 

 it is lined by endoderm, supported externally by the splanchnic sheet 



