552 MATURATION chap, m 



Maturation of the ovum. — The fully-formed ovum as 

 described on p. 546, is, however, incapable of being 

 fertilized or of developing into an embryo : before it is ripe 

 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 consists essentially in a twice-repeated process 

 of cell-division. The nucleus (Fig. 140, 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. Nuclear division then takes place, 

 one of the daughter nuclei remaining in the bud (/(?/), 

 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. 



In some cases development from an unfertilized 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) — Ijeing distinguished as parthenogenesis. It has been proved in 

 many instances and is probably generally true that in such cases the egg 

 begins to develop after the formation of the first polar cell. Thus in 

 parthenogenetic ova it appears that maturation is completed by the 

 separation of a single polar cell, after which the ovum contains the 

 number of chromosomes normal to the species. 



In the majority of cases, development takes place 

 only after fertilization, and in these maturation is not 

 complete until a second polar cell (b, pot) 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- 



