MATURATION AND IMPREGNATION OF THE OVUM, 121 



regarded as provisional, and I trust that this will be borne in 

 mind by the reader in perusing the following paragraphs. 



There is abundant evidence that at the time of maturation of 

 the egg the germinal vesicle undergoes peculiar changes, which 

 are, in part at least, of a retrogressive character. These changes 

 may begin considerably before the egg has reached the period of 

 maturity, or may not take place till after it has been laid. They 

 consist in appearance of irregularity and obscurity in the outline 

 of the germinal vesicle, the absorption of its membrane, the partial 

 absorption of its contents in the yolk, and the breaking upand disap- 

 pearance of the germinal spot. The exact fate of the single germinal 

 spot, or the numerous spots where they are present, is still obscure; 

 and the observations of Oellacher on the trout, and to a certain ex- 

 tent my own on the skate, tend to show that the membrane of the 

 germinal vesicle may in some cases be ejected from the egg, but 

 this conclusion cannot be accepted without further confirmation. 



The retrogressive metamorjjhosis of the germinal vesicle is 

 followed in a large number of instances by the conversion of 

 what remains into a striated spindle similar in character to a 

 nucleus previous to division. This spindle travels to the surface 

 and undergoes division to form the polar cell or cells in the 

 manner above described. The part which remains in the egg 

 forms eventually the female pronucleus. 



The germinal vesicle has up to the present time only been ob- 

 served to undergo the above series of changes in a certain 

 number of instances, which, however, include examples from 

 several divisions of the Coelenterata, the Echinodermata, and the 

 Mollusca, and also some of the Vermes (Nematodes, Hirudinea, 

 Sagitta). It is very possible, not to say probable, that it is uni- 

 versal in the animal kingdom, but the present state of our know- 

 ledge does not justify us in saying so. It may be that in the 

 case of the rabbit, and many Nematodes as described by van 

 Beneden and by Blitschli, we have instances of a different mode 

 of formation of the polar cells. 



The case of Amphibians, as described by Bambeke (2) and 

 Hertwig (12) cannot so far be brought into conformity with our 

 type, though observations are so difficult to make with such 

 opaque eggs that not much reliance can be placed upon the exist- 

 ing statements. In both of these types of possible exceptions it 

 is fairly clear that, whatever may be the case with reference to 

 the formation of the ])olar cells, part of the germinal vesicle 

 remains behind as the female pronucleus. 



There are a large number of types, including the whole of the 

 Rotifera' and ArtLropoda, with a few doubtful exceptions, in which 



' Flemming (6) finds that, in the summer and probably parthenogenetic 

 eggs of Lacinularia socialis, the germinal vesicle approaches the surface 



