xii MATURATION 569 



for conjugation with a sperm or able to undergo the first 

 stages of segmentation it has to go through a process known 

 as maturation. 



Maturation of the ovum consists essentially in a twice 

 repeated process of cell-division, and thus resembles the 

 process just described in the case of the sperms. The 

 nucleus (Fig. 150, A), which as we have seen contains 

 double the normal number of chromosomes, 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. Nuclear division then takes place, 

 without a splitting of the chromosomes, one of the daughter 

 nuclei remaining in the bud (pol\ the other in the ovum 

 itself. Then follows as usual a division of the protoplasm, 

 and the bud becomes separated as a small cell distinguished 

 as the first polar cell. 



In some cases development from an unfertilised female gamete takes 

 place, the process which is not uncommon among insects (e.g. the 

 common little green plant-louse or Aphis] and crustaceans (e.g. water- 

 fleas) being distinguished as parthenogenesis. It has been proved that 

 in the majority of such cases the egg begins to develop after the forma- 

 tion of the first polar cell, when the ovum contains the number of 

 chromosomes normal to the species. 



In the majority of cases, development takes place 

 only after fertilisation, and in these maturation is not 

 complete until a reducing division (p. 566) has occurred in 

 the formation. of a second polar cell (B, pol}. 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 becomes enclosed in a nucleus, 

 which is distinguished as the female pronncleus (B, ^prori). 



