ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 329 



appear separate, owing to the creuate form of the mass. 

 Against this may be put the fact that the division of a 

 nucleus is by no means so common as has been sometimes 

 supposed, that in segmentation it has very rarely been 

 observed that the nucleus of a sphere first divides/ 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 distinct 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 specimens of this age which 

 show the formation of these new cells with great clearness. 

 This is most distinctly to be seen immediately below the em- 

 bryo, 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 certain of 

 this. 



' Kowalevsky (" Beitrage zur Entwickelungsgeschichte der Holothurien," 

 'Memoirs de I'Ac. Imp. de St. Petersbourg,' vii ser., vol. xi, 1867) describes 

 tlie division of nuclei during segmentation in the Eolotburians, and other 

 observers have described it elsewhere. 



