144 



EMBRYOLOGY. 



Fig. 109. 



unite to form the cavity of the body, is very unlike in dif- 

 ferent animals ; and these several modes 



are of high importance in classification. 

 Among the Vertebrates, the embryo lies 

 with its face or ventral surface towards 

 the yolk, (Fig. 109,) and thus the suture, 

 or line at which the edges of the germ 

 unite to enclose the yolk, and which in 

 the mammals forms the navel, is found 

 in front. Another suture is found along 

 the back, arising from the actual folding upwards of the 

 upper surface of the germ, to form the dorsal cavity. 



304. The embryo in the Articulata, on the contrary, lies 

 with its back upon the yolk, as seen in the following figure, 



which represents an embryo of Podurella ; 

 consequently the yolk enters the body on 

 that side ; and the suture, which in the 

 vertebrates is found on the belly, is here 

 found on the back. In the Cephalopoda 

 the yolk communicates with the lower 

 side of the body, as in Vertebrates, but 

 there is no dorsal cavity formed in them. 

 In the other Mollusks, as also in the Worms, there is this 

 peculiarity, that the whole yolk is changed at the beginning 

 into the substance of the embryo ; whilst in Vertebrates, and 

 the higher Articulates and Mollusks, a part of it is reserved, 

 till a later period, to be used for the nourishment of the em- 

 bryo. Among Radiata, the germ is formed around the yolk, 

 and seems to surround the whole of it, from the first.* 



305. The development of the embryo of the vertebrated 

 animals may be best observed in the eggs of fishes. Being 



Fig. 110. 



* These facts show plainly that the circumstance of embryos arising 

 from the whole or a part of the yolk is of no systematic importance. 



