MATURATION OF THE OVUM. 



73 



originate by a regular process of cell-division and have the value 

 of cells. 



A peculiar phenomenon makes its appearance in the eggs of Clepsinc 

 shortly after the formation of the polar cells, which has been spoken of by 

 Whitman (No. 100) as the formation of the polar rings. The following is his 

 description of the occurrence. 



" Fifteen minutes after the elimination of the polar globules (i.t. cells) a 

 ring-like depression or constriction appears in the yolk around the oral pole, 

 and in this depression a transparent liquid substance (nuclear ?) is collected 

 forming the first polar ring.... The same phenomena repeat themselves 

 lateral the aboral pole.. ..The rings concentrate to form two discs.... Before 

 the first cleavage both discs plunge deep into the egg." 



The nature of these rings is at present quite obscure. 



Considering how few ova have 

 been adequately investigated with 

 reference to the behaviour of the 

 germinal vesicle, any general con- 

 clusions which may at present be 

 formed are to be regarded as pro- 

 visional. 



There is however abundant 

 evidence that at the time of matu- 

 ration 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 an appearance of irregularity and obscurity 

 in the outline of the germinal vesicle, the absorption of its mem- 

 brane, the partial absorption of its contents in the yolk, the dis- 

 appearance of the reticulum, and the breaking up and disappear- 

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

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



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 of the 

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



FIG. 30. OVUM OF ASTERIAS 

 GLACIALIS WITH THE TWO POLAR 



CELLS AND THE FEMALE PRONUCLEUS 

 SURROUNDED BY RADIAL STRI., AS 

 SEEN IN THE LIVING EGG (copied from 



Fol). 



