174 GENERAL BIOLOGY 



and carrying them out through the tissues of the body wall 

 to the surface and thus, removing them. The nephridia 

 develop from the mesoderm, also, but the greater part of 

 the mesodermal cells develop into muscle fibres. They 

 elongate greatly, and acquire a structure especially fitting 



KARL ERNST VON BAER 



(1792-1876) 

 The founder of comparative embryology. 



them for contracting in one direction. The tissue that over- 

 spreads the walls of the ccelom (formed, as already seen, by 

 the splitting of the mesoderm) is the peritoneum. It con- 

 sists for the most part of thin flat covering cells, but 

 where these cells overlie the stomach-intestine they become 

 distended with waste assimilation products and take on an 



