60 POLAR CELLS. 



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 of the ovum 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 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 universal 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 Beiieden (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, arid 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 

 .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 bodv and the other does not stain but is 



