INTRODUCTION 



11 



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

 fertilization have been worked 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 

 is therefore reasonably 



certain that in all cases A 



each daughter 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 still 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 Mollusca. 

 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 Dinophilus according 

 to Shearer (1912), the 

 spermatozoon enters the 

 oogonium and remains 

 passive during the growth 



FIG. 4. Two stages in the first division of the fertilized 

 egg of Crepidula plana. (After Conklin.) 



A, the first cleavage spindle ; female chromosomes above 

 separated by an interval from male chromosome below. B, the 



and maturation of the 

 germ cell. 



If the eggS are Stale, division of the zygote nucleus is complete. /, Female chromo- 

 ' p if fhpv bavp hppn somes ; TO, male chromosomes ; pi (in A), first polar body, (in B), 



products of division of first polar body ; j^, second polar body. 



shed too long ironi the 



ovary before being fertilized, 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 fertiliza- 

 tion, extra achromatic fibres can be developed and irregular division 



