FOURTH PERIOD OF DEVELOPMENT. 



Period of the histological differentiation. 

 JUST as the third period of development may be 

 described as that in which the differentiation of the 

 organs follows from the two primary germinal layers, 

 so the histological differentiations which concern these 

 organs may be described as the most important pro- 

 cesses of the fourth period of development. Especial 

 account is to be taken of the formation of the muscles, 

 the histological differentiation of the notochord, and 

 of the fibres in the neural canal. 



The formation of new organs is in this period of 

 development only of secondary importance. The two 

 evaginations of the alimentary canal, which we pre- 

 viously mentioned, are completely separated from 

 the alimentary canal, and are altered in a remarkable 

 way. There is further originated a gland which 

 was noticed in older larvae by Leuckart and Pagen- 

 stecher, and minutely described by Kowalevsky. 

 The formation of the ventral blood-vessel now begins. 



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