128 THE AMPHIOXUS. 



of a cylindrical, to that of a cubical epithelium, only 

 on the anterior point of the body, and on the posterior 

 end the cells remain columnar. 



The cilia, which, as already mentioned, every cell 

 bears, grow in the course of development to a con- 

 siderable length, as has been represented in Fig. 50. 



ABSORPTION OF THE YOLK GRANULES. 



The absorption of the yolk granules, and there- 

 with the transparency of the embryos, make con- 

 tinual advance during this period of development. 



In general, the cells of the posterior end of the body 

 contain in all layers more abundant yolk granules 

 than those in the remaining parts. 



"With regard to the layers of the body, it may be 

 said that the yolk granules attain their absorptions 

 most quickly in the outer epithelium, where they 

 at the end of this period of development are now 

 not to be found by any means plentifully. The 

 mesoblast follows next, then the medullary plate, 

 and the hypoblast last, so far as the speed is con- 

 cerned with which the yolk granules attain their 

 absorption (compare the sections on Table VIII.). 



