206 COMPARATIVE ZOOLOGY. 



Birds, segmentation is confined to the small white speck 

 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 gas- 

 trula 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 construc- 

 tion. 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 medullary fur- 

 row the mark of the coming Vertebrate. The blasto- 

 dermic layer of the Vertebrate egg rolls up into two tubes 

 one to hold the viscera, the other to contain the nervous 

 cord; while that of the Invertebrate egg forms only one 

 such tubular division. The features which determine the 

 subkingdom to which an animal belongs are first devel- 

 oped, then the characters revealing its class. 



There are differences also in grade of development as 

 well as type. For a time there is no essential difference 

 between a Fish and a Mammal : they have the same ner- 

 vous, circulatory, and digestive systems. There are many 

 such cases, in which the embryo of an animal represents 

 the permanent adult condition of some lower form. In 

 other words, the higher species, in the course of their de- 

 velopment, offer likenesses, or analogies, to finished lower 

 species. The human germ, at first, cannot be distinguished 

 from that of any other animal : for aught we can see, it 

 may turn out a Frog or a Philosopher. The appearance 

 of a medullary furrow excludes it at once from all Inver- 

 tebrates. It afterwards has, for a time, structures found in 

 the lower classes and orders of Vertebrates as permanent 



