REPRODUCTION 



93 



now exposed to very different conditions ; in consequence 

 they assume different characteristics. The cells in the 

 outer layer, in contact with sea water, become more or 



Pig. 57.— Diagram of a gaetrula. 



less hardened and assume a protective function. Those 

 on the inside assume a nutritive function. We thus 

 have formed two primitive layers called the ectoderm 

 and the entoderm. As the cells increase in number 

 a new layer called the mesoderm is formed between 

 them. These are the three primary layers. From them 

 all the tissues of the adult organism are formed. The 

 higher the animal the more marked the power of cellular 

 differentiation, and the more highly developed the re- 

 sulting tissues. From the ectoderm all protective organs 

 are formed, skin, nervous system, sense organs ; out of 

 the entoderm, all organs which have to do with alimenta- 

 tion, the alimentary canal, the liver, pancreas, lungs; 



