1 3 o THE AMPHIOXUS. 



Finally, at the end of this period of development, 

 preparation is made for the perforation of the mouth 

 and first gill-slit, and of the anus. 



Although the important part of this period of 

 development depends upon the histological differen- 

 tiation, yet the most striking appearance is the con- 

 siderable change in the outer form, i.e. of the larva. 

 The ovoid shape was in the former period of develop- 

 ment somewhat altered through elongation and 

 lateral compression, without, however, leading to 

 a characteristically expressed form of body. In the 

 period of development, however, which is now before 

 us, the body, through considerable elongation, receives 

 continued lateral compression. Through the growing 

 out of the epiblast cells of the posterior end to a 

 caudal fin, and through nose-like elongation of the 

 anterior end of the body, it acquires a fish-like form, 

 which vividly reminds us of the vertebrate animal 

 type (Table V., Figs. 54-61). 



THE FORMATIONS OF THE MESOBLAST. 

 We will now review the alterations which concern 

 the mesoblast formations. 



Kowalevsky's observations have yielded but little 



