SEGMENTATION. 22/ 



segments are, as a rule, larger than the more central ones, though 

 in this respect there is considerable irregularity. The segments 

 are becoming smaller by repeated division ; but, in addition to 

 this mode of increase, there is now going on outside the 

 germinal disc a segmentation of the yolk, by which fresh seg- 

 ments are being formed from the yolk and added to those which 

 already exist in the germinal disc. One or two such segments 

 are seen in the act of being formed (PI. 6, fig. 7 /) ; and it is to 

 be noticed that the furrows which will eventually mark out the 

 segments, do so at first in a partial manner only, and do not 

 circumscribe the whole circumference of the segment in the act 

 of being formed. These fresh furrows are thus repetitions on a 

 small scale of the earliest segmentation furrows. 



It deserves to be noticed that the portion of the germinal 

 disc which has already undergone segmentation, is still sur- 

 rounded by a broad band of small-sized yolk-spherules. It 

 appears to me probable that owing to changes taking place in 

 the spherules of the yolk, which result in the formation of fresh 

 spherules of a small size, this band undergoes a continuous 

 renovation. 



The uppermost row of segmentation spheres is now com- 

 mencing to be distinguished from the remainder as a separate 

 layer which becomes progressively more distinct as segmenta- 

 tion proceeds. 



The largest segments in this section measure about the 

 -ji^th of an inch in diameter, and the smallest about ^jth of 

 an inch. 



The nuclei at this stage present points of rather a special in- 

 terest. In the first place, though visible in many, and certainly 

 present in all the segments 1 , they are not confined to these: 

 they are also to be seen, in small numbers, in the band of 

 fine spherules which surrounds the already segmented part of 

 the germinal disc. Those found outside the germinal disc are 

 not confined to the spots where fresh segments are appearing, 



1 In the figure of this stage, I have inserted nuclei in all the segments. In the 

 section from which the figure was taken, nuclei were not to be seen in many of the 

 segments, but I have not a question that they were present in all of them. The 

 difficulty of seeing them is, in part, clue to the yolk -spherules and in part to the 

 thinness of the section as compared with the diameter of a segmentation sphere. 



