FIRST PERIOD OF DEVELOPMENT. 53 



ones, were formed of about thirty-two. In most cases, 

 however, the regular arrangement of the tiers had 

 b3en lost through shifting of the calls, while their 



o o / 



number was diminishing. 



Despite the loss of the tiers, the chief axis 

 drawn from the upper to the lower pole can still be 

 clearly distinguished in these stages. 



Indeed, all about the lower pole, a region of some- 

 what large dark cells can be distinguished, which 

 for the greatest part at any rate have their origin 

 in the eight cells of the lower pole, and are plainly 

 different from the other and smaller cells. 



The smallest cells are to be found at the upper 

 pole. 



During this period the transition to the blastula 

 stage is already noticeable, while the dimension of 

 the cells in height gradually surpasses the other 

 dimensions. The cells, too, through a flattening of 

 the ends, first those turned towards the segmenta- 

 tion cavity, and afterwards those outside, acquire an 

 epithelial-like character. 



During these processes, which are accompanied by 

 a constant increase of the cells, there takes place 

 a continual growth of the segmentation cavity. This 



