142 THE AMPHIOXUS. 



at the conclusion of the former period of development. 

 This separation and removal from the wall of the 

 mesenteron is completed at the end of the period of 

 development which we have now been considering. 

 "With the trunk-like outgrowth of the anterior end 

 of the body is united an elongation of this part of 

 the notochord. The notochord appears here to grow 

 more extensively than the neighbouring tissues. Its 

 anterior point seems quite wedged in between the epi- 

 blast cells of the end of the body ; through excessive 

 growth it often experiences here swelling or curving. 



The histological differentiation of the notochord 

 is introduced by the appearance of numerous vacuoles 

 at first small in the interior of the notochord cells. 

 The vacuoles are especially numerous in the middle 

 cells ; in the dorsal and ventral cell series of the 

 notochord their number is very small. These little 

 vacuoles make their appearance as early as the end 

 of the former period of development in embryos with 

 nine to ten mesoblastic somites (Fig. 50). 



The vacuoles become continually larger, and their 

 number in consequence smaller. This is to be ex- 

 plained by the fact that several small ones coalesce 

 together. 



