228 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 for- 

 mation of fresh segments outside the germinal disc and with 

 other facts which I shall mention hereafter, is of great morpho- 

 logical interest as bearing upon the nature and homologies of 

 the food-yolk. It also throws light upon the behaviour and 

 mode of increase of the nuclei. All the nuclei, both those of the 

 segments and those of the yolk, have the peculiar structure I 

 described in the last stage. 



In specimens of this stage I have been able to observe 

 certain points which have an important bearing upon the be- 

 haviour of the nucleus during cell-division. 



Three figures, illustrating the behaviour of the nucleus, as I 

 have seen it in sections of blastoderms hardened in chromic acid, 

 are shewn in PL 6, figs. 7 a, 7 b and 7 c. 



In the place of the nucleus is to be seen a sharply defined 

 figure (Fig. 7 a) stained in the same way as the nucleus or more 

 deeply. It has the shape of two cones placed base to base. 

 From the apex of each cone there diverge towards the base a 

 series of excessively fine striae. At the junction between the 

 two cones is an irregular linear series of small deeply stained 

 granules which form an apparent break between the two. The 

 line of this break is continued very indistinctly beyond the edge 

 of the figure on each side. 



From the apex of each cone there diverge outwards into the 

 protoplasm of the cell a series of indistinct markings. They are 

 rendered obscure by the presence of yolk-spherules, which 

 completely surround the body just described, but which are not 

 arranged with any reference to these markings. These latter 

 striae, diverging from the apex of the cone, are more distinctly 

 seen when the apex points to the observer (Fig. 7 b), than when 

 a side of the cone is in view. 



The striae diverging outwards from the apices of the cones 

 must be carefully distinguished from the striae of the cones 

 themselves. The cones are bodies quite as distinctly differ- 

 entiated from the protoplasm of the cell as nuclei, while the 

 striae which diverge from their apices are merely structures in 

 the general protoplasm of the cell. 



