MATURATION OF THE EGG 313 



as in the preceding paragraph to designate chromosomes 

 then two cells may be formed with paternal (a, b, c, d) 

 and maternal (A, B, C, D) respectively, or these may 

 exist in combination (A, b, c, d and a, B, C, D, etc.). 

 With eight chromosomes sixteen combinations are pos- 

 sible; with forty-eight as in man the possible number 

 runs into the millions. This random assortment, 

 as it is termed, results in a wide variation as will 

 be learned in a subsequent chapter and is of the highest 

 importance in connection with the evolution of a species. 



Maturation of the Egg. — As a result of the accumu- 

 lation of food substances during the growth period, 

 each egg mother cell becomes a relatively large body; 

 but while it is thus superficially unlike the correspond- 

 ing small sperm cell its history during the maturation 

 period is the same. It undergoes a reduction division 

 and there is the same random assortment of the chromo- 

 somes, but the four cells formed as a result of the two 

 maturation divisions are very unlike in size. This merely 

 means, in the light of their subsequent behavior, that 

 the three small cells without yolk (Fig. 75) and incapable 

 of fertilization, are rudimentary eggs, while the remaining 

 large cell alone is capable of development. Four func- 

 tional sperms develop from one mother cell, while one 

 functional and three vestigial eggs are derived from the 

 egg mother cell. 



Determination of Sex. — Up to the present time it 

 has been assumed that the number of chromosomes in the 

 body cells of the male and female is the same, but at 

 least one hundred species of animals are known where 

 the male has one or at most a very few less than the 

 female. This occasions a modification of the maturation 

 divisions. Let it be assumed that the body cells of the fe- 

 male have eight chromosomes and the male seven (Fig. 

 77) — as actually happens in some species. The history 

 of the maturation divisions of the egg is the same as that 

 already described. In the male, on the other hand, the 



