66 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



passive yolk-spheres imbedded in it. The organized basis is 

 especially concentrated at the germinal pole of the egg, but 

 becomes less and less in quantity, as compared with the yolk- 

 spheres, the further we depart from this. 



Admitting, as I think it is necessary to do, the organized 

 condition of the whole yolk-sphere, there are two possible views 

 as to its nature. We may either take the view that it is one 

 gigantic cell, the ovum, which has grown at the expense of the 

 other cells of the egg-follicle, and that these cells in becoming 

 absorbed have completely lost their individuality; or we may 

 look upon the true formative yolk (as far as we can separate it 

 from the remainder of the food-yolk) as the remains of one cell 

 (the primitive ovum), and the remainder of the yolk as a body 

 formed from the coalescence of the other cells of the egg-follicle, 

 which is adherent to, but has not coalesced with, the primitive 

 ovum, the cells in this case not having completely lost their 

 individuality ; and to these cells, the nuclei, I have found, must 

 be supposed to belong. 



The former view I think, for many reasons, the most pro- 

 bable. The share of these nuclei in the segmentation, and the 

 presence of similar nuclei in the cells of the germ, both support 

 it, and are at the same time difficulties in the way of the other 

 view. Leaving this question which cannot be discussed fully in 

 a preliminary paper like the present one, I will pass on to 

 another important question, viz. : 



How do these nuclei originate ? Are they formed by the 

 division of the pre-existing nuclei, or by an independent for- 

 mation? It must be admitted that many specimens are strongly 

 in favour of the view that they increase by division. In 

 the first place, they are often seen "two together;" examples 

 of this will be seen in PI. 3, fig. i. In the second place, 

 I have found several specimens in which five or six appear 

 close together, which look very much as if there had been an 

 actual division into six nuclei. It is, however, possible in 

 this case that the nuclei are really connected below and only 

 appear separate, owing to the crenate 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 ob- 



