252 DEVELOPMENT OF ELASMOBRANCH FISHES. 



One of the most important differences between the two is 

 that of size. The majority of the nuclei present in the yolk are 

 as large or larger than an ordinary blastoderm cell ; while many 

 of them reach a size very much greater than this. The examples 

 I have measured varied from -gfo to -^ of an inch in diameter. 



Though they are divided, like the nuclei of the blastoderm, 

 with more or less distinctness into separate areas by a network 

 of lines, their greater size frequently causes them to present an 

 aspect somewhat different from the nuclei of the blastoderm. 

 They are moreover much less regular in outline than these, and 

 very many of them have lobate projections (PI. 7, figs. 2 a and 

 2c and 3), which vary from simple knobs to projections of such a 

 size as to cause the nucleus to present an appearance of com- 

 mencing constriction into halves. When there are several such 

 projections the nucleus acquires a peculiar knobbed figure. With 

 bodies of this form it becomes in many cases a matter of great 

 difficulty to decide whether or no a particular series of knobs, 

 which appear separate in one plane, are united in a lower plane, 

 whether, in fact, there is present a single knobbed nucleus or a 

 number of nuclei in close apposition. A nucleus in this con- 

 dition is represented in PL J, fig. 2 b. 



The existence of a protoplasmic network in the yolk has 

 already been mentioned. This in favourable cases may be 

 observed to be in special connection with the nuclei just de- 

 scribed. Its meshes are finer in the vicinity of the nuclei, and 

 its fibres in some cases almost appear to start from them (PI. 7, 

 fig. 12). For reasons which I am unable to explain the nuclei 

 of the yolk and the surrounding meshwork present appearances 

 which differ greatly according to the reagent employed. In 

 most specimens hardened in osmic acid the protoplasm of the 

 nuclei is apparently prolonged in the surrounding meshwork 

 (PI. 7, fig. 12). In other specimens hardened in osmic acid 

 (PI. 7, fig. 11), and in all hardened in chromic acid (PI. 7, fig. 2a 

 and 2c), the appearances are far clearer than in the previous 

 case, and the protoplasmic meshwork merely surrounds the nuclei, 

 without shewing any signs of becoming continuous with them. 



There is also around each nucleus a narrow space in which 

 the spherules of the yolk are either much smaller than else- 

 where or completely absent, vide PI. 7, fig. 2b. 



