54 THE AMPHIOXUS. 



growth of the segmentation cavity appears to take 

 place at the expense of the cell mass, which becomes 

 somewhat smaller. 



THE BLASTULA (Fias. 19, 20). 



The transition from the segmentation stages to 

 that stage which we designate by the name blastula 

 is characterized in the following way. The cells, 

 which had formerly approached the spherical form, 

 being therefore flattened against one another only to a 

 limited extent, and which as well outwards as inwards 

 pushed forward towards the segmentation cavity with 

 a decided tendency to being spherical, now lie against 

 one another, and acquire a more epithelial-like character. 

 In the next place epiblast cells, which compose the 

 upper two-thirds of the roof, are changed, while first 

 its inner surface, that, namely, which is turned to- 

 wards the segmentation cavity, and then, finally, its 

 outer surface are altered in the characteristic way 

 (Figs. 16 and 20). 



It is not till somewhat later that the lower third, 

 composed of larger and darker cells representing 

 the hypoblast, is affected by similar processes. The 

 cells had hitherto kept a certain, and for the seg- 



