266 



DEVELOPMENT OF ELASMOBRANCH FISHES. 



fig. 14. In full-sized ovarian eggs, according to Schultz 1 , it 

 forms, as was mentioned in the first chapter, radiating striae, 

 extending from the centre to the periphery of the egg. When 

 examined with the highest powers, the lines of this network 

 appear to be composed of immeasurably small granules arranged 

 in a linear direction. These granules are more distinct in chromic 

 acid specimens than in 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. 252) touched upon the relation of this 

 network to the nuclei of the yolk 2 . 



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 PL 7, fig. 7, and 

 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 (PL 7, fig. 13). Although it would 

 be possible for cells in the living state to pass from the blasto- 

 derm 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, (i) That the cells 



1 Archivfiir Mikr. Anal. Vol. xi. 



2 A protoplasmic network resembling in its essential features the one just de- 

 scribed 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. (JenaischeZeitschrift, Vol. vu.) Metschnikoff (Zeitschrift f. Wiss. Zoologie, 

 1874) nas demonstrated its presence in the ova of many Siphonophorias and Medusae. 

 Flemming (" Entwicklungsgeschichte der Najaden," Site, derk. 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 fiir Mikr. 

 Anat., Vol. vin.) in the ovarian ova of Reptiles. Eimer moreover finds that it is 

 continuous with prolongations from cells of the epithelium of the follicle in which 

 the ovum is contained. According 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. 



