DEVELOPMENT OF ELA.SMOBRANCH FISHES. 15 



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 segments 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. II. 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 circum- 

 ference 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 surrounded 

 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 

 jl^th of an inch in diameter, and the smallest about s^o^h 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 ^ 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, 

 but are also to be seen in places where there are no traces of 

 fresh segments. 



This fact, especially when taken in connection with the forma- 



^ 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, due to the yolk-spherules and in part to 

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

 sphere. 



