MATURATION AND IMPREGNATION OF THE OVUM. 539 



It is possible that the removal of part of the protoplasm of 

 the egg in the formation of the polar cells may be a secondary 

 process due to an attractive influence of the nucleus on the cell 

 protoplasm, such as is ordinarily observed in cell division. 



Impregnation of the Ovum. 



A far greater amount of certainty appears- to me to have been 

 attained as to the effects of impregnation than as to the changes 

 of the germinal vesicle which precede this, and there appears, 

 moreover, to be a greater uniformity in the series of resulting 

 phenomena. For convenience I propose to reverse the order 

 hitherto adopted and to reserve the history of the literature and 

 my discussion of disputed points till after my general account. 

 Fol's paper on Asterias glacialis, is again my source of informa- 

 tion. The part of the germinal vesicle which remains in the egg, 

 after the formation of the second polar cell, becomes converted 

 into a number of small vesicles (Fig. 10), which aggregate them- 

 selves into a single clear nucleus which gradually travels toward 

 the centre of the egg and around which as a centre the protoplasm 

 becomes radiately striated (Fig. n). This nucleus is known as 

 \.\\Q female pronnclcus 1 . In Asterias glacialis the most favourable 

 period for fecundation is about an hour after the formation of 

 the female pronucleus. If at this time the spermatozoa are 

 allowed to come in contact with the egg, their heads soon 

 become enveloped in the investing mucilaginous coat. A pro- 

 minence, pointing towards the nearest spermatozoon, now arises 

 from the superficial layer of protoplasm of the egg and grows 

 till it comes in contact with the spermatozoon (Figs. 12 and 13). 

 Under normal circumstances the spermatozoon, which meets the 

 prominence, is the only one concerned in the fertilisation, and it 

 makes its way into the egg by passing through the prominence. 

 The tail of the spermatozoa, no longer motile, remains visible for 

 some time after the head has bored its way in, but its place is 

 soon taken by a pale conical body which is, however, probably 

 in part a product of the metamorphosis of the tail itself (Fig. 14). 

 This body vanishes in its turn. 



1 According to Hertwig's most recent statement a nucleolus is present in this 

 nucleus. 



352 



