io8 THE AMPHIOXUS. 



the transverse section of the cells are seen, of which 

 about four comprise the thickness of the notochord. 



There may, indeed, be observed a lower and an 

 upper series of cells, and about two less regularly 

 arranged middle series (Fig. 50). 



In this stage, also, the histological differentiation of 

 the notochord begins to be expressed by the appear- 

 ance of numerous small vacuoles. We will return to 

 this point on a closer consideration of the histological 

 differentiations, which are included in the next period 

 of development. 



We have now still to explain in what manner the 

 advance of the notochord proceeds backwards and 

 also towards the anterior end. The changes we have 

 described of the dorsal hypoblast fold take place in 

 the stages from the formation of the sixth to that of 

 the tenth mesoblastic somite, and in general it may 

 be said that the differentiations proceed more quickly 

 in the neighbourhood of the anterior mesoblastic 

 somites. For instance, while in the stage with six 

 mesoblastic somites, in the anterior mesoblastic 

 somites the median slit of- the notochord foundation 

 has already vanished (Figs. 99, 100), the latter is 

 still preserved in its entirety in the last but one, 



