DEVELOPMENT 



ing the body into successive rings, or joints. Next, we 

 observe that the back lies near the center of the egg, the 

 ventral side looking outward, i.e., the embryo is doubled 

 upon itself backward. And, finally, the appearance of 

 three pairs of legs proves that it will be an insect, rather 

 than a worm, crustacean, or spider. 



The vertebrate embryo lies with its stomach toward 

 the yolk, reversing the position of the articulate ; but 

 the grand characteristic is the medullary groove, which 

 does not exist in the egg of any invertebrate. This 

 feature is connected with another, the setting apart of 

 two distinct regions the nervous and nutritive. There 

 are three modifications of vertebrate development : that 

 of fishes and amphibians, that of true reptiles and 

 birds, and that of mammals. The amnion and allantois 

 are wanting in the first group ; while the placenta (which 

 is the allantois vitally connected with the parent) is pe- 

 culiar to mammals. In mammals, the whole yolk is seg- 

 mented ; in birds, segmentation is confined to the small 

 white speck (blastoderm] seen in opening the shell. 



At the outset, all animals, from the sponge to man, 

 are structurally alike. All, moreover, undergo segmen- 

 tation, and most have one form or other of the gastrula 

 stage. But while vertebrates and invertebrates can 

 travel together on the same road up to this point, here 

 they diverge never to meet again. For every grand 

 group early shows that it has a peculiar type of con- 

 struction. Every egg is from the first impressed with 

 the power of developing in one direction only, and never 

 does it lose its fundamental characters. The germ of 

 the bee is divided into segments, showing that it belongs 

 to the articulates ; the germ of the lion has the medul- 

 lary furrow the mark of the coming vertebrate. The 

 blastodermic layer of the vertebrate egg rolls up into 

 two tubes one to hold the viscera, the other to con- 

 DODGE'S GEN. ZOOL. 27 



