74 POLAR CELLS. 



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 Ccelenterata, the Echinodermata, and the 

 Mollusca, some of the Vermes [Turbellarians (Leptoplana], 

 Nematodes, Hirudinea, Alciope, Sagitta], Ascidians, etc. It is 

 very possible, not to say probable, that such changes are univer- 

 sal in the animal kingdom, but the present state of our knowledge 

 does not justify us in saying so. 



In the Craniata especially our knowledge of the formation of the polar 

 bodies is very unsatisfactory. In Petromyzon Kupffer and Benecke have 

 brought forward evidence to shew that one polar body is formed prior to 

 the impregnation, and a second in connection with a peculiar prominence 

 of protoplasm after impregnation. Part of the germinal vesicle remains in 

 the egg as the female pronucleus. In the Sturgeon the germinal vesicle 

 atrophies and breaks up before impregnation, and afterwards part is found as a 

 granular mass on the surface of the egg, while part forms a female pronucleus. 



In Amphibia the observations of Hertwig (90) and Bambeke (77) tend to 

 shew that after the germinal vesicle has assumed a superficial situation at 

 the pigmented pole of the ovum its contents become intermingled with the 

 yolk, and are in part extruded from the ovum as a granular mass after 

 impregnation. Part of them remains in the ovum and forms a female 

 pronucleus. Whether there is a proper division of the germinal vesicle 

 as in typical cases is not known. 



Oellacher (95) by a series of careful observations upon the egg of the trout, 

 and subsequently of the bird, demonstrated that in the ovum while still in 

 the ovary, the germinal vesicle underwent a kind of degeneration and 

 eventually became ejected, in part at any rate. My own observations on 

 Elasmobranchs, which require enlargement and confirmation, tend to shew 

 that this part may be the membrane. Ed. van Beneden (78) has contributed 

 some important observations on the rabbit. His account is as follows. As 

 the ovum approaches maturity the germinal vesicle assumes an eccentric 

 position, and fuses with the peripheral layer of the egg to constitute the 

 cicatricular 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 plasma of the nucleus 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 



