FORMATION OF THE EMBRYO 1081 



Formation of the Embryo. Not till all these events have taken place 

 extrusion of the two polar bodies, or maturation ; penetration of the 

 spermatozoon, and blending of its head (the male pronucleus) with the 

 remnant of the nucleus of the ovum (female pronucleus), or fecundation 

 not till then does the ovum begin the process of repeated division by 

 which the whole body is reproduced. The fused or segmentation nucleus 

 divides into two, each containing the normal number of chromosomes 

 derived from the splitting of those contributed by both the male and 

 female elements. It is believed that the division takes place in such a 

 way that both male and female chromosomes are represented in each 

 nucleus. The cytoplasm being also cleft by a corresponding furrow, 

 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 protovertebrte. 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- 

 pleure is a space called the ccelom, or pleuro-peritoneal cavity (Fig. 462). 

 The 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 activity is buried beneath the surface of the blastoderm, and 



