112 F. M. BALFOUR. 



stantiated, or that all the questions connected with these pheno- 

 mena are settled. The statements we have are in many points 

 opposed and contradictory, and there is much that is still very 

 obscure. 



In the sequel an account is first given of the researches of the 

 above-named authors, followed by a statement of those results 

 which appear to me the most probable. 



The researches of van Beneden (3 and 4) were made on the 

 ovum of the rabbit and of Asterias, and from his observations 

 on both these widely separated forms he has been led to con- 

 clude that the germinal vesicle is either ejected or absorbed, 

 but that it has in no case a genetic connection with the first seg- 

 mentation sphere. He gives the following description of the 

 changes in the rabbit's ovum. The germinal vesicle is enclosed by 

 a membrane, and contains one main germinal spot, and a few acces- 

 sory ones, together with a granular material which he calls nucleo- 

 plasma, which affects, as is usual in nuclei, a reticular arrange- 

 ment. The remaining space in the vesicle is filled by a clear fluid. 

 As the ovum approaches maturity the germinal vesicle assumes 

 an excentric position, and fuses with the peripheral layer of the 

 egg to constitute the cicatricnlar lens. The germinal spot next 

 travels to the surface of the cicatricular lens and forms the 

 nuclear disc : at the same time the membrane of the germinal 

 vesicle vanishes though it probably unites with the nuclear disc. 

 The nucleoplasma then collects into a definite mass and forms 

 the nucleoplasmic body. Finally the nuclear disc assumes 

 an ellipsoidal form and becomes the nuclear body. Nothing is 

 now left of the original germinal vesicle but the nuclear body 

 and the nucleoplasmic body both still situated within the ovum. 

 In the next stage no trace of the germinal vesicle can be 

 detected in the ovum, but outside it, close to the point where the 

 modified remnants of the vesicle were previously situated, there 

 is present a polar body which is composed of two parts, one of 

 which stains deeply and resembles the nuclear body, and the other 

 does not stain but is similar to the nucleoplasmic body. Van 

 ]3eneden concludes that the polar bodies are the two ejected 

 products of the germinal vesicle. In the case of Asterias, 

 van Beneden has not observed the mode of formation of the 

 polar bodies, and mainly gives an account of the atrophy of the 

 germinal vesicle, but adds very little to what was already known 

 to us from Kleinenberg's {ii) earlier observations. He describes 

 with precision the breaking up of the germinal spot into frag- 

 ments and its eventual disappearance. 



Though tliere are reasons for doubting the accuracy of all the 

 above details on the ovum of the rabbit, nevertheless, the observa- 

 tions of van Beneden taken as whole afford strong grounds for 



