BIRDS' EGGS 253 



§ 3. Maturation of the Ovum 



In the middle of the drop of formative protoplasm on the 

 top of the yolk lies the relatively large nucleus, containing 

 readily stainable nuclear rods or chromosomes, definite in 

 number for each species. These are the vehicles of parts at 

 least of the hereditary equipment. 



At a certain stage, while still in the ovary, the egg-cell 

 undergoes a remarkable process called maturation. The 

 nucleus shows signs of activity ; its membrane disappears, 

 admitting of a freer give-and-take between the nucleoplasm 

 and the general cytoplasm of the cell ; and it divides twice 

 in rapid succession, forming two polar bodies — minute 

 sister cells of the ovum that come to nothing. One result of 

 this maturation is that the number of chromosomes in the 

 nucleus is reduced to one half of the normal number, 

 probably in the formation of the first polar body. This is 

 called a " reducing division " or " meiotic division," and 

 it is very different from the ordinary cell- division that goes 

 on continually in developing and growing structures. 



If we compare a nucleus with its chromosomes to a box 

 of wooden matches we may think of two possible ways of 

 halving these with precision. Half of the matches may be 

 taken out and put into a separate box — this would corre- 

 spond to reducing division. Or each match might be split 

 into two longitudinally, so that the number of useable 

 matches remains the same, the two sets of split halves being 

 put in separate boxes. This is what takes place in ordinary 

 or equation division. As a similar reduction takes place in 

 the history of the male's germ-cells or spermatozoa, it is 

 plain that in fertilisation the normal number will be restored, 



n n 



- + - = w. So it is evident that the maturation divisions 

 2 2 



of the egg-cell mean, in part, a preparation for fertiUsation. 

 To this it is now necessary to turn. 



