282 EMBETOLOGT. 



tion by which its edges approach each other, and unite to form 

 the cavity of the body, is very unlike in 

 Fig. 295. different animals ; and these several modes 



are of high importance in classification. 

 Among the vertebrata, the embryo lies with 

 its face or ventral surface towards the yolk 

 (fig. 295), and thus the suture, or line at 

 which the edges of the germ unite to en- 

 close the yolk, and which in the mam- 

 mals forms the navel, is found in front. 

 Another suture is found along the back, 

 arising from the actual folding upwards of the upper sur- 

 face of the germ, to form the dorsal cavity. 



461. The embryo in the articulata, on the contrary, lies 

 Fig. 296 w ^k i ts 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 vertebrata is found on 

 the belly, is here, as also in the worms, found 

 on the back. In the cephalopoda the 

 yolk communicates with the lower side of 

 the body as in the vertebrata, but there is no dorsal cavity 

 formed in them. In the other mollusca there is this peculiarity, 

 that the whole yolk is changed at the beginning into the sub- 

 stance of the embryo ; whilst in the vertebrata and the higher 

 articulata and mollusca, a part of it is reserved, till a later 

 period, to be used for the nourishment of the embryo. Among 

 the radiata the germ is formed around the yolk, and seems to 

 surround the whole of it, from the first.* 



462. The development of the embryo of vertebrated 

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

 transparent, they do not require to be cut open, and by suffi- 

 cient caution, the whole series of embryonic changes may be 

 observed upon the same individual, and thus the succession 

 in which the organs appear, may be ascertained with precision ; 

 whereas, if we employ the eggs of birds, which are opaque, 

 we are obliged to sacrifice an egg for each observation. 



463. To illustrate these general views as to the develop- 



* These facts show that the circumstance of embryos arising from the 

 whole or a part of the yolk is of no systematic iinportance. 



