EMBRYOLOGICAL HISTORY. 157 



a fair statement to say that there is not a group of 

 animals, with the possible exception of the sponges, 

 which does not show in its development some traces 

 of this gastrula. It is true that the gastrula is 

 formed in many ways ; it is true that it has a very 

 different appearance in different cases ; that it is in 

 some cases only dimly suggested ; it is true that its 

 original method of formation is still a matter of 

 dispute, and that the relations of its various parts to 

 the adult are not in all cases settled ; it even ap- 

 pears under different names in some of its different 

 forms (planula plakula). But these points only 

 make the case the stronger, for in spite of the vari- 

 ous modifying circumstances affecting the develop- 

 ment, so strong is the tendency to inherit this stage, 

 that it is very difficult to find even general excep- 

 tions to the rule. The significance of this fact can 

 hardly be overrated. For the embryologist it is suf- 

 ficient proof that if we could correctly follow back 

 the history of animals, we could trace them all back 

 to a common ancestor which in fundamental features 

 resembled this two-layered sac. And this represents 

 the first multicellular animal which the evolution 

 theory assumes ever to have existed. The stage 

 before this was not a multicellular animal, but a 

 colony of independent unicellular individuals. But 

 now the independence of the different cells is lost. 

 They become united into a dependent unity. Some 

 of the cells have one duty to perform for the whole 

 colony, and others have a different function. One set 

 of cells does all of the digesting, and another has 

 the function of sensation, etc. A division of labor 



