INTEODUCTION 



11 



dula plana, a species in which the processes of maturation and 

 fertilization have been worlced out in great detail by Conklin 

 (1902). The centrosomes of the first cleavage spindle are here 

 stated to^be derived from the division of the sperm centrosome. It 

 reasonably 



IS therefore 

 certain that m 

 each daughter 



all cases 

 -.giiuv^jL cell re- 

 ceives a half of each male 

 and female chromosome. 



It is by no means 

 always true that the 

 spermatozoon can only 

 enter the egg after the 

 formation of both polar 

 bodies. In many cases 

 it enters the egg whilst 

 it is stiU an oocyte of 

 the first order, and even 

 before the nuclear mem- 

 brane has been dissolved 

 and the germinal spot 

 has disappeared. This is 

 true of the eggs of many 

 Annelida and MoUusca. 

 In other cases, such as 

 in some Ascidians, the 

 first maturation spindle 

 is formed before the 

 spermatozoon enters, but 

 the first maturation 

 division is not completed 

 till the spermatozoon is 

 inside the egg. Finally, 

 in Binophilus according 

 to Shearer (1912), the 

 spermatozoon enters the 

 oogonium and remains 

 passive during the growth 

 and maturation of the 

 germ cell. 



If the eggs are stale, 

 i.e. if they have been 

 shed too long from the 

 ovary before being fertihzed, then more than one spermatozoon can 

 enter them, and an extra centrosome is thus introduced, between 

 which and one or both of the centrosomes resulting from the division 

 of the centrosome of that sperm which has actually effected fertihza- 

 tion, extra achromatic fibres can be developed and irregular division 



Fig. 4. — Two stages in the first division of the fertilized 

 egg of Crepidula plana. (After Conklin.) 



A, the first cleavage spindle ; female cliromosomes above 

 separated by an interval from malejCbromosome below. B, the 

 division of the zygote nucleus is complete. /, Female chromo- 

 somes ; m, male chromosomes ; pi (in A), first polar body, (in B), 

 products of division of iirst polar body ; p'^, second polar body. 



