MATURATION AND IMPREGNATION OF THE OVUM. 535 



General conclusions. 



Considering how few ova have been adequately investigated 

 with reference to the behaviour of the germinal vesicle any 

 general conclusions which may at present be formed are to be 

 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 break- 

 ing up and disappearance 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 Oellacheron 

 the trout, and to a certain extent my own on the skate, tend to 

 shew 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 metamorphosis 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 

 observed 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 



