LESS. XXIII POLAR CELLS 259 



c, the head of the sperm has become the male pronucleus ( i pron), 

 its intermediate piece the male centrosome ( 6 cent) ; other structures as 

 before. 



D, the male and female pronuclei are in the act of conjugation. 



E, conjugation is complete and the segmentation nucleus (seg. mut) 

 formed. (From Parker and Ilaswell's Z.oology.) 



Maturation consists essentially in a twice-repeated process 

 of cell-division. The nucleus (Fig. 62, a,) loses its mem- 

 brane, travels to the surface of the egg, and takes on the 

 form of an ordinary nuclear spindle. Next the protoplasm 

 grows out into a small projection or bud, into which one end 

 of the spindle projects. The usual process of nuclear 

 division then takes place (Fig. 10, p. 64), one of the 

 daughter nuclei remaining in the bud {pol), the other in 

 the ovum itself. Nuclear division is followed as usual by 

 division of the protoplasm, and the bud becomes separated 

 as«a small cell distinguished as the first polar cell. 



It was mentioned in a previous lesson (p. 200) that in 

 some cases development from an unfertilized female gamete 

 took place, the process — which is not uncommon among 

 insects and crustaceans — being distinguished as partheno- 

 genesis. It has been proved in many instances and may be 

 true in the majority of cases that the egg begins to develop 

 after the formation of the first polar cell. Thus in many 

 parthenogenetic ova maturation is completed by the separa- 

 tion of a single polar cell. 



In the majority of animals, however, development takes 

 place only after fertilization, and in such cases maturation is 

 not complete until a second polar cell(^, pol) has been formed 

 in the same manner as the first. The ovum has now lost a 

 portion of its protoplasm together with three-fourths of its 

 chromatin, half having passed into the first polar cell and 

 half of what remained into the second : the remaining one- 

 fourth of the chromatin takes on a rounded form and is dis- 

 tinguished as the female pronucleus (b, $ pron). 



s 2 



