INTRA-OVARIAN EGG OF SOME OSSEOUS FISHES. 61 



process of the nucleus. Balfour 1 writes on this subject: 

 " Yolk-spherules arise as extremely minute, highly-refracting 

 particles in a stratum of protoplasm some little way below the 

 surface, and are always most numerous at the pole opposite the 

 germinal vesicle." " It deserves to be specially noted that 

 when the yolk-spherules are first formed, the peripheral layer of 

 the ovum is entirely free from them." " Two points about 

 the spherules appear clearly to point to their being developed 

 in the protoplasm of the ovum, and not in the follicular epi- 

 thelium. 1. That they do not make their appearance in the 

 superficial stratum of the ovum. 2. That no yolk-spherules 

 are present in the cells of the follicular epithelium, in which 

 they could not fail to be detected." Balfour's opinion is that 

 Gegenbaur's 2 account of the formation of the yolk-spherules in 

 birds is probably correct. " Protoplasmic molecules," says 

 Gegenbaur, grow into larger granules. These become larger 

 again, and are transformed into vesicles, which increase con- 

 tinuously in volume. New solid products now take their origin 

 in the interior, and dissolve into fine granules." " The yolk- 

 spherules originate in the interior of the egg, and not from the 

 follicular epithelium." These observations, which were made 

 in birds', agree with those in reptiles' eggs. 



On the other hand, yolk-spherules appear also sometimes to 

 be formed from without. At any rate, 3 Beddard regards it as 

 highly probable. His view, which was deduced from the study 

 of the ovarian ovum of Lepidosiren (Protopterus), is supported, 

 among other facts, by their being no external membrane in this 

 stage. The follicle lies immediately on the yolk, and masses 

 of migrating cells were seen in course of being budded off 

 from it. 



To return to the nucleus again ; it rapidly degenerates as 



1 Balfour, F., " Structure aud Development of the Vertebrate Ovum," 

 • Quart. Journ. Micr. Sci.,' vol. xviii, 1878. 



2 Gegenbaur, " Ueber d. Bau uud d. Entwicklung d. Wirbelthier Eier mit 

 partieller Dottertheilung," ' Midler's Archiv,' 1861. 



1 Beddard, P., " Observations on the Ovarian Ovum of Lepidosiren," 

 4 Proceedings of the Zoolog. Soc.,' 1886. 



