ORGANIC EVOLUTION 195 



and the formation and subsequent withdrawal from the 

 surface of the yolk plug. The segmentation cavity is 

 reduced in size as the endodermal sac increases in depth. 



Then the mesoderm appears in the space between ecto- 

 derm and endoderm at the blastopore on the upper side, and 

 spreads outward and downward in a thin sheet of tissue, and 

 begins to split to form a coelom. Up to this point in 

 development it will be observed that nothing has appeared 

 to suggest a vertebrate animal. In the one celled stage the 

 embryo is more like a protozoan in structural type ; in the 

 blastula stage it has the hollow spherical form of volvox; 

 in the gastrula stage it is more like hydra in plan. As soon 

 as it has acquired a compound tubular body through the 



Fig. 120. Sections of salamander eggs in a meridional plane, 

 8-cell, 32-cell and later segmentation stages, 



development and splitting of a mesodermal layer, it is more 

 like a worm, and not until the appearance of a central 

 nervous axis upon the dorsal side is there a single structure 

 present that can be pointed out as distinctively characteristic 

 of a backboned animal. Thus the series of embryonic 

 forms assumed by the salamander in its development shows 

 a rough correspondence to the series of adult forms we have 

 been studying. Furthermore, when vertebrate characters 

 appear they are very generalized indeed, and the parts in 

 formatibn look no more like the adult salamander than like 

 other vertebrate animals. 



Vertebrate characters. — Several distinctively vertebrate 

 characters appear now in different parts of the body almost 



