226 DEVELOPMENT OF ELASMOBRANCH FISHES. 



nuclei in many of the segments, though it is very difficult even 

 to see them, and only in very favourable specimens can their 

 structure be studied. In some cases, two of them lie one on 

 each side of a furrow; and in one case at the extreme end of a 

 furrow I could see two peculiar aggregations of yolk-spherules 

 united by a band through which the furrow, had it been con- 

 tinued, would have passed. The connection (if any exists) be- 

 tween this appearance and the formation of the fresh nuclei 

 in the segments, I have been unable to elucidate. 



The peculiar appearances attending the formation of fresh 

 nuclei in connection with cell-division, which have recently 

 been described by so many observers, have hitherto escaped my 

 observation at this stage of the segmentation, though I shall 

 describe them in a later stage. A nucleus of this stage is 

 shewn on PI. 6, fig. 6 c. It is lobate in form and is divided by 

 lines into areas in each of which a deeply-stained granule is 

 situated. 



The succeeding stages of segmentation present from the 

 surface no fresh features of great interest. The somewhat 

 irregular (PI. 6, figs. 4 and 5) circular line, which divides the 

 peripheral larger from the central smaller segments, remains for 

 a long time conspicuous. It appears to be the representative of 

 the horizontal furrow which, in the Batrachian ovum, separates 

 the smaller pigmented spheres from the larger spheres of the 

 lower pole of the egg. 



As the segments become smaller and smaller, the distinction 

 between the peripheral and the central segments becomes less 

 and less marked; but it has not disappeared by the time that 

 the segments become too small to be seen with the simple 

 lens. When the spheres become smaller than in the germinal 

 disc represented on PI. 6, fig. 5, the features of segmentation 

 can be more easily and more satisfactorily studied by means of 

 sections. 



To the features presented in sections, both of the latter and 

 of the earlier blastoderms, I now return. A section of one of 

 the earlier germinal discs, of about the age of the one represented 

 on PI. 6, fig. 4, is shewn in PI. 6, fig. 7. 



It is clear at a glance that we are now dealing with true seg- 

 ments completely circumscribed on all sides. The peripheral 



