154 EDOUARD VAN BENEDEN. 



identical conclusions on certain questions of capital im- 

 portance, and at very different results with regard to other, 

 equally fundamental points. 



Among the problems which he and I have solved very 

 differently are, in the first place, the history of the germinal 

 vesicle ; and secondly, the question of the formation of the 

 first embryonic nucleus. 



My researches on the ovum of the rabbit have proved to 

 me that no morphological part of the germinal vesicle is found 

 in the yolk at the moment of fecundation. The nucleolus 

 united with the substance which constituted the membrane 

 of the vesicle is eliminated to form one of the " directive 

 bodies ;" the nucleoplast with the pseudo-nucleoli are thrown 

 off into the perivitelline liquid, to form there the second polar 

 globule. The liquid of the vesicle remains in the yolk, and 

 becomes confounded with the cortical substance of the ovum, 

 which from this moment is no longer distinguishable from 

 the medullary substance. There cannot then be, in the 

 rabbit, any genetic connection between the germinal vesicle 

 or one of its parts, and the embryonic nucleus which appears 

 in the egg after fecundation. I have moreover been able to 

 observe all the phases in the formation of the latter. The 

 first nucleus is developed at the expense of a body formed in 

 the cortical layer of the ovum, which I have called the 

 periplieral pronucleus y and of another body which appears in 

 the centre of the yolk, and which I have called the central 

 pronucleus. It is probable that the first embryonic nucleus 

 is not formed by the fusion of the two pronuclei : — the 

 peripheral pronucleus, at first smaller tlian the other, en- 

 larges at the expense of the central. The latter becomes 

 attached to the former, and then its substance becomes 

 absorbed by it, as, I think, by a process of endosmosis. 



According to the observations of M. Hertwig on the Toxo- 

 pneustes lividus, things take place in a difierent way. When the 

 germinal vesicle has quitted the centre of the ovum to place 

 itself under the membrane, and has, so to speak, left the yolk, 

 the germinal spot in its turn forsakes the germinal vesicle, 

 penetrates into the yolk, and becomes what the author calls 

 the nucleus of the ovum (JEikern) ; the germinal vesicle then 

 undergoes retrograde metamorphosis ; its membrane becomes 

 dissolved, and the rest is finally absorbed by the yolk. 



As to the formation of the first nucleus of the embryo, 

 which he calls nucleus of the first cleavage-sphere, or more 

 simply, cleavage nucleus, M. Hertwig has ascertained that it 

 is the product of the copulation of two nuclei. In from five 

 to ten minutes after the sperm has been mixed with the eggs. 



