112 



EMBRYOLOGY. 



298. At this early epoch, namely, a few days, and, in 



some animals, a 

 few hours after 

 development has 

 begun, the germ 

 Fig. 102. Fig. 103. consists of a sin- 



gle layer composed of very minute cells, all of them having 

 the same appearance and the same form (Fig. 102, g). But 

 soon after, as the germ increases in thickness, several layers 

 may be discerned (Fig. 103), which become more and 

 more distinct. 



299. The upper layer (s), in which are subsequently 

 formed the organs of animal life, namely, the nervous 

 system, the muscles, the skeleton, &c. (59), has received 

 the name of serous or nervous layer. The lower layer (in], 

 which gives origin to the organs of vegetative life, and 

 especially to the intestines, is called the mucous or vegeta- 

 tive layer, and is generally composed of larger cells than 

 those of the upper or serous layer. Finally, in the em- 

 bryos of vertebrated animals, there is a third layer (v), 

 interposed between the two others, and giving rise to the 

 organs of circulation and to the blood ; whence it has 

 been called blood layer, or vascular layer. 



300. Even before this epoch, we can generally distin- 

 guish, from the manner in which the germ is modified, to 

 what department of the animal kingdom the individual is to 

 belong. Thus in the Articulata, the germ is divided into 



segments, indicating the rings of the 

 body, as for example, in the embryo 

 of the crabs (Fig. 104). The germ 

 of the vertebrated animals, on the 

 other hand, displays a longitudinal 

 Fig. 104. Fig. 105. furrow, which marks the position 

 the future back-bone is to occupy (Fig. 105). 



