MATURATION AND IMPREGNATION OF THE OVUM. 525 



vesicle assumes an excentric position, and fuses with the peri- 

 pheral layer of the egg to constitute the cicatriciilar 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 nucleo- 

 plasmic body. Van Beneden 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 (14) 

 earlier observations. He describes with precision the breaking 

 up of the germinal spot into fragments and its eventual dis- 

 appearance. 



Though there are reasons for doubting the accuracy of all the 

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

 vations of van Beneden taken as a whole afford strong grounds 

 for concluding that the formation of the polar cells is connected 

 with the disappearance, partial or otherwise, of the germinal 

 vesicle. A very similar account of the apparent disappearance 

 of the germinal vesicle is given by Greeff (19) who states that 

 the apparent disappearance of the germinal spot precedes that 

 of the vesicle. 



The observations of Biatschli are of still greater importance in 

 this direction. He has studied with a view to elucidating the 

 fate of the germinal vesicle, the eggs of Nephelis, Lymnaeus, 

 Cucullanus, and other Nematodes; and Rotifers. In all of these, 

 with the exception of Rotifers, he finds polar bodies, and in this 



