552 MATURATION chap. \i 



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 inatiiratioii. 



Maturation consists essentially in a twice-repeated process 

 of cell-division. The nucleus (Fig. r4o, a) loses its mem- 

 Ijrane, 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. 



lit s<;nnc c;iscs <lcvi-'lo[imcnl (I'Din an linlcfLilizcd feiiiaW y;iiincLo lakes 

 jilacc, Llic pi'oocss — \\liicli is not uiiconinion aiiKHig insects (t'.^'. lliu 

 commun little gieeii plant-lcjiise or Apliii) and crustaceans (e.g. water- 

 fleas) — being distinguished -.x^ parljwiiogriu'si:^. It has been pru\ cd in 

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

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

 ])arthcnogenetic 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 majorit)- of cases, de\eloi)mcnt takes place 

 only after fertili/^ation, and in these maturation is not 

 loinplete until a scunid polar uil (i:., pol) has been formed 

 in the sarne manner as the first. Tlie ovum has now lost a 

 portion of its protoplasm together with three-fourths of its 

 chromatin, half ha\ing passed into the first polar cell ;ind 

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



