1 14 EMBRYOLOGY. 



303. In all classes of the Animal Kingdom, the embryo 



rests upon the yolk, and covers it like a 

 cap. But the direction by which its edges 

 approach each other, and unite to form the 

 cavity of the body, is very unlike in differ- 

 ent animals ; and these several modes 

 are of high importance in classification. 

 Fi?. 109. 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 at the belly. 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 from 



the opposite direction ; and the suture, 

 which in the vertebrates is found on the 

 belly, is here found on the back. In the 

 Mollusks there is this peculiarity, that the 

 whole yolk is changed into the substance Fig. no. 

 of the embryo ; whilst in Vertebrates, a part of it is re- 

 served, till a later period, to be used as food by the embryo. 

 Among Radiata the germ is formed around the yolk, and 

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



305. Among the vertebrated animals, the development of 

 the embryo may be best observed in the eggs of fishes. 

 Being transparent, they do not require to be cut open, and, 

 by sufficient caution, we may observe the whole series of 

 changes upon the same individual, and thus make sure of 

 the succession in which the organs appear ; whereas, if we 

 employ the eggs of birds, which are opaque, we are obliged 

 to sacrifice an egg for each observation. 



