THIRD PERIOD OF DEVELOPMENT. 99 



in the first mesoblastic somite, from which a con- 

 tinuation as a mesoblast formation of the head (sensu 

 strictiore) grows into the anterior end of the body. 

 There we have, as specially characteristic of this 

 epoch, the formation of the notochord, of the neural 

 canal, and finally of the anterior outgrowths of the 

 alimentary canal which are then gradually abstricted 

 from one another. 



We already know the alteration of the body-form 

 in general through Kowalevsky's investigations. The 

 alterations which the mesoblastic somites experience 

 in this epoch have been indicated by him only cur- 

 sorily and in part. The formation of the notochord 

 we only know through his work in so far as the 

 same was proved to take its origin from the hypo- 

 blast as a dorsal canal derived from it. "We will 

 return to Kowalevsky's statements and deal with 

 them more fully when explaining the developmental 

 conditions of the individual organs. 



f 



FORMATION OF THE NOTOCHORD. 



We will now describe more closely that which 

 characterises this section, namely the formation of 

 the notochord. 



