Hubbard.] "^ [Dec. 15. 



exactly at the entodermic pole of the egg. It seems to tend to that defin- 

 ite position when it first leaves the nucleus. But without any question it 

 takes up its station there before the nucleus begins to move towards the 

 periphery of the egg, as the nucleus always moves in a directly opposite 

 direction from the body. In all eggs the yolii collects at the entodermic 

 pole of the egg. Does this peculiar body serve as an attraction for the 

 yolk in this egg? Or is it a mere coincidence that it is in the midst of the 

 yolk and has no particular connection with it? 



In conclusion, I would say that I cannot definitely determine its func- 

 tion. I think the body is homologous with the meganucleus in proto- 

 zoans. It is the vegetative portion of the egg given otf from the nucleus 

 when the egg cell has become fully differentiated as such. That wlricli 

 remains of the nucleus is homologous with the micronucleus in proto- 

 zoans. It is the animal part of the egg which is further concerned wholly 

 with the reproduction of the species. 



Historical Review of the Liteuatuke on the Yolk Nucleus. 



Hoffman ('78, p. 545) in his studies on the young ovarian eggs of am- 

 phibians saw the yolk nucleus in Raaa esculentia. He merely mentions 

 it and describes it as a round, dark, granular body within the yolk. lie 

 says it is not seen in all amphibians. 



Balbiani ('79) has had some very peculiar views on the origin of this 

 body. His ideas being now so untenable, I quote this more as a curiosity 

 than as shedding any light on the true origin. He considers the follicular 

 cells as homologous to the spermatoblasts. The yolk nucleus corresponds 

 to the spermatic elements. One, becoming free from the follicle, penetrates 

 the yolk. Wlien it first enters it leaves a sort of canal behind it, which is 

 soon closed up by the surrounding yolk. It is a sort of spermatozoid and 

 partially fertilizes the egg preliminary to the true fertilization which takes 

 place later. In parthenogenetic eggs this body would perform the func- 

 tion of the male element. If his descriptions are based on facts the body 

 he described is not homologous with the yolk nucleus of authors in 

 general. 



Schiifer ('80) describes and figures this body in the eggs of the rabbit. 

 He thinks he saw some connection between the nucleus and the yolk 

 nucleus. He believes that the latter is derived from the former, but not 

 by a process of ordinary cell division. 



Schutz ('82) has given an extended review of this body as seen in vari- 

 ous classes of animals. But he seems to have no adequate conception 

 either of its origin, function or fate. He remarks that the yolk may have 

 produced it and that it later serves as nutrition for the yolk. The observa- 

 tions were made on isolated specimens of fresh eggs. No sections were 

 made nor was the body traced out in any single species. He has, how- 

 ever, carefully compiled a large list of animals in which it has been seen, 

 and has described its structure and appearance in many of them. His refer- 



