DEVELOPMENT OF ELASMOBRANCH FISHES. 53 



those hardened in osmic acid, but are to be seen in both. 

 There can be little doubt that these granules are imbedded 

 in a thread or thin layer of protoplasm. 



I have already (p. 39) touched upon the relation of this 

 network to the nuclei of the yolk\ 



During the stages which have just been described specially 

 favourable views are frequently to be obtained of the formation 

 of cells in the yolk and their entrance into the blastoderm. 

 Two representations of these are given, in PI. iv. fig. 7 or, 

 and PL v. fig. 13. In both of these distinctly circumscribed 

 cells are to be seen in the yolk (c), and in all cases are situated 

 near to the typical nuclei of the yolk. The cells in the yolk 

 have such a relation to the surrounding parts, that it is quite 

 certain that their presence is not due to artificial manipulation, 

 and in some cases it is even difficult to decide whether or no a 

 cell area is circumscribed round a nucleus (PI. v. fig. 13). 

 Although it would be possible for cells in the living state to 

 pass from the blastoderm into the yolk, yet the view that they 

 have done so in the cases under consideration has not much to 

 recommend it, if the following facts be taken into consideration. 

 (1) That the cells in the yolk are frequently larger than those 

 in the blastoderm. (2) That there are present a very large 

 number of nuclei in the yolk which precisely resemble the 

 nuclei of the cells under discussion. (3) That in some cases 

 (PL V. fig. 13) cells are seen indistinctly circumscribed as if in 

 the act of being formed. 



Between the blastoderm and the yolk may frequently be 

 seen a membrane-like structure, which becomes stained with 

 hgematoxylrn, osmic acid etc. It appears to be a la3^er of 

 coagulated albumen and not a distinct membrane. 



1 A protoplasmic network resembling in its essential features the one just 

 described has been noticed by many observers in other ova. Fol has figured 

 and described a network or sponge-like arrangement of the protoplasm in the 

 eggs of Geryonia. (Jenaische Zeitschrift, vol. vii.) Metschnikoff [Zcitschrift f. 

 Wiss. Zoologie, 1874) has demonstrated its presence in the ova of many Sipho- 

 nophoriae and Medusas. Flemming {Entwicklungsgeschichte der Najaden, Sitz. 

 der k. Akad. Wien, 1875) has found it in the ovarian ova of fresh-water mussels 

 (Anodonta and Unio), but regards it as due to the action of reagents, since he 

 fails to find it in the fresh condition. Amongst vertebrates it has been carefully 

 described by Eimer {Archiv fur Mikr. Anat., vol. viii.) in the ovarian ova of 

 Eeptiles. Eimer moreover finds that it is continuous with prolongations from 

 cells of the epithehum of the follicle in which the ovum is contained. Accord- 

 ing to him remnants of this network are to be met with in the ripe ovum, but 

 are no longer present in the ovum when taken from the oviduct. 



