CELL DIVISION AND GAMETOGENESIS 



235 



In pigeons, it has been found that the female-producing egg 

 is not only larger than the male-producing egg but also contains 

 a greater amount of chemical potential energy. Ordinarily 

 pigeons lay eggs in clutches of two, one of which on the average 

 develops into a male and the other into a female. It has been 

 discovered that if eggs are removed from the nest as fast as laid, 

 the female is stimulated to lay a greater total number of eggs 

 than normally and that as a result the percentage of female- 

 producing eggs is increased. Presumably the overstimulation 

 caused by enforced egg-laying activity led to the production of 

 eggs of greater metabolic capacity. The normal sex ratio of the 

 offspring is changed by changing the metabolic rate of the 

 mother. 



Delaying fertilization in frog eggs for three or four days 

 increases the number of males. Frog tadpoles reared at a 

 temperature of 32°C. produce a pre- 

 ponderance of males. These and 

 many other experiments might be 

 cited to illustrate the fact that the 

 zygotic sex may be changed by factors 

 that in some way disturb the metabolic 

 balance that normally accompanies, or 

 is correlated with, a certain combina- 

 tion of chromosomes. 



Chromatin Diminution. — As a gen- 

 eral rule, the zygotic number of 

 chromosomes, established at fertiliza- 

 tion, continues to be the number found 

 in the somatic cells of the embryo and later of the adult, and 

 also the number occurring in the presynaptic stages of the 

 germ cells. An exception to this generalization is the phe- 

 nomenon of chromatin diminution, as it occurs in certain species 

 of nematode worms belonging to the genus Ascaris. In Ascaris 

 megalocephala bivalens, the zygotic number of chromosomes is 4. 

 In the first and second cleavages of the fertilized egg, each of these 

 large V- or U-shaped chromosomes is divided longitudinally 

 in the usual way, so that each of the four cells receives 4 chromo- 

 somes. In the late prophase stages of the next cell division 

 (third cleavage) in each of three cells the ends of the chromosomes 

 are cast off into the cytoplasm and the chromosomes that actually 



S.C. 



Fig. 144. — Third cleavage of 

 egg of Ascaris megalocephala, 

 semidiagrammatic. s.c, stem 

 cell. 



