DEVELOPMENT OF THE ELASMOBRANCH FISHES. 67 



served that the nucleus of a sphere first divides 1 , and that 

 then segmentation takes place, but segmentation generally 

 occurs and then a new nucleus arises in each of the newly 

 formed spheres. Such nuclei as I have described are rare ; 

 they have, however, been observed in the egg of a Nephelis 

 (one of the Leeches), and have in that case been said to 

 divide. Dr Kleinenberg, however, by following a single egg 

 through the whole course of its development, has satisfied 

 himself that this is not the case, and that, further, these nuclei 

 in Nephelis never form the nuclei of newly developing cells. 



I must leave it an open question, and indeed one which can 

 hardly be solved from sections, whether these nuclei arise freely 

 or increase by division, but I am inclined to believe that both 

 processes may possibly take place. In any case their division 

 does not appear to determine the segmentation or segregation 

 of the protoplasm around them. 



As was mentioned in my account of the segmentation, these 

 nuclei first appear during that process, and become the nuclei 

 of the freshly formed segmentation spheres. At the close of 

 segmentation a few of them are still to be seen around the 

 blastoderm, but they are not very numerous. 



From this period they rapidly increase in number, up to the 

 commencement of the formation of the embryo as a body dis- 

 tinct from the germ. Though before this period they probably 

 become the nuclei of veritable cells which enter the germ, it is 

 not till this period, when the growth of the blastoderm becomes 

 very rapid and it commences to spread over the yolk, that these 

 new cells are formed in large numbers. I have many speci- 

 mens of this age which shew the formation of these new cells 

 with great clearness. This is most distinctly to be seen imme- 

 diately below the embryo, where the yolk-spherules are few 

 in number. At the opposite end of the blastoderm I believe 

 that more of these cells are formed, but, owing to the presence of 

 numerous yolk-spherules, it is much more difficult to make cer- 

 tain of this. 



1 Kowalevsky (" Beitrage zur Entwickelungsgeschichte der Holothurien," Me- 

 moirs de I' Ac. Imp. de St Petersbourg, vii sen, Vol. XT. 1867) describes the division 

 of nuclei during segmentation in the Holothurians, and other observers have described 

 it elsewhere. 



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