142 



EMBRYOLOGY. 



of very minute cells, all of which are alike in appearance 

 and form, (Fig. 102, g.) But soon after, as the germ increases 

 in thickness, several layers may be discerned, in vertebrated 

 animals, (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 sys- 

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

 name of serous or nervous layer. The lower layer, (m,) 

 which gives origin to the organs of vegetative life, and espe- 

 cially to the intestines, is called the mucous or vegetative 

 layer, and is generally composed of larger cells than those 

 of the upper or serous layer. Finally, there is a third 

 layer, (v,) interposed between the two others, giving rise to 

 the formation of blood and the organs of circulation ; whence 

 it has been called blood layer, or vascular layer. 



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

 can generally distinguish, at a very early epoch, to what de- 

 partment of the animal kingdom an individual is to belong. 

 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. 104.) 

 The germ of the vertebrated ani- 

 mals, on the other hand, displays 

 a longitudinal furrow, which marks 

 the position the future back-bone 

 is to occupy, (Fig. 105.) 



301. The development of this furrow is highly important, 

 as indicating the plan of structure of vertebrated animals in 

 general, as will be shown by the following figures, which 

 represent vertical sections of the embryo at different epochs.* 



Fig. 104. 



Fig. 105. 



* In these figures, the egg is supposed to be cut down through the mid- 

 dle, so that only the cut edge of the embryo is seen ; -whereas, if viewed 



