280 



EMBEYOLOGT. 



to the formation of a germ comprising the whole yolk, or 

 rising above it as a disc-shaped protuberance, composed of 

 little cells, which has been variously designated under the names 

 of germinative disc, proligerous disc, blastoderma, germinal 

 membrane. In this case, however, that portion of the yolk 

 which has undergone less obvious changes, forms nevertheless 

 part of the growing germ. The disc again enlarges, until it 

 embraces the whole, or nearly the. whole, of the yolk. 



455. At this early epoch, aa.iiely, a few days, and, in 



Fig. 288. Fig. 289. S0me animals a 



few hours after de- 

 velopment has be- 

 gun, the germ pro- 

 per consists of a 

 single layer com- 

 posed of very mi- 

 nute cells, all of them alike in appearance and form (fig. 

 2^8, g). But soon after, as the germ increases in thickness, 

 several layers may be discerned in vertebrate animals (fig. 

 289), which become more and more distinct. 



456. The upper layer (*), in which are subsequently 

 formed the organs of animal life, namely, the nervous system, 

 the muscles, the skeleton, &c. ( 76), has received the name 

 of serous or nervous layer. The lower layer (ni), which gives 

 origin to the organs of vegetative life, and especially to the 

 intestines, is called the mucous or vegetative layer, and is 

 generally composed of cells larger than those of the upper or 

 serous layer. Finally, in the embryos of vertebrated animals, 

 there is a third layer (v), interposed between the two others, 

 giving rise to the formation of the blood and the organs of cir- 

 culation ; whence it has been called the blood layer or vascular 

 layer. 



457. From the manner in which the germ is modified, we 

 can generally distinguish, at a very 

 early epoch, to what department of the 

 animal kingdom an individual is to be- 

 long. Thus in the articulata, the germ 

 is divided into segments, indicating the 

 transverse divisions of the body, as, for 

 example, in the embryo of the crabs 

 (fig. 290). The germ of the verte- 

 brated animals, on the other hand, displays a longitudinal fur- 



Fig. 290. 



Fig. 291. 



