MATURATION OF THE OVUM. 



59 





which is clearly derived from part of the original germinal vesicle is call- 

 ed the femalepronucleus, for reasons which will appear in the sequel. 



The two polar cells appear to be situated 

 between two membranes, the outer of which 

 is very delicate, and only distinct where it 

 covers the polar cells, while the inner one is 

 thicker and becomes, after impregnation, more 

 distinct, and then forms what Fol speaks of 

 as the vitelline membrane. It is clear, as 

 Hertwig has pointed out, that the polar bodies 

 originate by a regular process of cell-division 

 and have the value of cells. 



FIG. 29. PORTION OF THE 

 OVUM OF ASTERIAS GLACTALIS 

 AFTER THE FORMATION OF THE 

 SECOND POLAK CELL, SHEWING 

 THE PART OF THE SPINDLE RE- 

 MAINING IN THE OVUM BE- 

 COMING CONVERTED INTO TWO 

 CLEAR VESICLES. PlCRIC ACID 

 PREPARATION (copied from 

 Fol). 



A peculiar phenomenon makes its appearance 

 in the eggs of Clepsine 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.e. cells) a 

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

 and in this depression a transparent liquid substance (nuclear 1) is collected 

 forming the first polar ring The same phenomena repeat themselves 



later at 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 conclu- 

 sions which may at present be formed 

 are to be regarded as provisional. 



There is however abundant evi- 

 dence that at the time of maturation 

 of the egg the germinal vesicle under- 

 goes peculiar changes, which are, in 

 part at least, of a retrogressive charac- 

 ter. These changes may begin con- 

 siderably 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 

 membrane, the partial absorption of its contents in the yolk, the 

 disappearance of the reticulum, and the breaking 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. 



The retrogressive metamorphosis of the germinal vesicle is followed 



FIG. 30. OVUM OF ASTEKIAS GLACI- 

 ALIS WITH THE TWO POLAR CELLS AND 

 THE FEMALE PRONUCLEUS SURROUNDED 

 BY RADIAL STRIDE, AS SEEN IN THE 

 LIVING EGG (copied from Fol). 



