FOURTH PERIOD OF DEVELOPMENT. 145 



or larvae. At the posterior end where the notochord 

 continues to grow and its tissues become again more 

 and more differentiated, all the stages of this differ- 

 entiation may be observed one after the other. We 

 may follow step by step the appearance of the 

 vacuoles, their elongation, their movement into one 

 single series, so that perpendicular walls of separation 

 are formed between them as notochord plates, to which 

 we add the alterations of the cells. Here, where we 

 see the stages of development near one another in 

 direct transition, we find the correctness of the above 

 statements established. 



The continuation of the histological differentiation 

 from, anterior to posterior is thus very sharply stamped 

 upon the notochord. 



The general picture of the histological differenti- 

 ation of the notochord already shows plainly enough 

 the same type which we find in the developed animal. 

 The transverse fibring too of the notochord plates 

 appears, as we shall soon see, very early. 



