416 TEXT-BOOK OF EMBRYOLOGY. 



Worms, and in one of the plant lice two forms of ova are produced, the larger of which 

 always develops into females and the smaller into males, is evidence in favor of the progam- 

 ous determination of sex. In the rotifer and the plant louse the development is partheno- 

 genetic and consequently the spermatozoa can have no influence. Furthermore, it has been 

 shown that the female rotifer, if well fed, will produce only the larger (female) ova, if poorly 

 fed, only the smaller (male) ova. The most remarkable facts in favor of progamous deter- 

 mination of sex have been discovered within the past few years by McClung, Wilson, Morgan, 

 Correns and others. These investigators have demonstrated variations in the number of 

 chromosomes in many species of Insects, the variation being constant in each species. In 

 nearly 60 species, Wilson has found two classes of spermatozoa, differing constantly in the 

 number of their chromosomes. Morgan ha's shown further that in certain forms of these in- 

 sects which produce a series of parthenogenetic generations, the fertilized ova produce 

 females only, and the parthenogenetic (non-fertilized) ova produce both males and females. 

 He has also shown that the production of females only is associated with the fact that the 

 spermatozoa with the fewer chromosomes are small and degenerate without reaching ma- 

 turity. This leaves only one kind of active spermatozoa that unite with the ova, and all the 

 fertilized ova produce females. In regard to the males and females produced without 

 fertilization, Morgan has made the discovery that the somatic cells of the males contain only 

 five chromosomes, whereas those of the females contain six. 



Since the researches have been carried on principally with Insects, there has been some 

 doubt as to whether the variations in the number of chromosomes might not obtain in this 

 order alone. But from the fact that Correns has demonstrated a like variation in the pollen 

 and ova of flowering plants, it seems not impossible that the phenomenon is of wide occurrence. 



The only evidence in favor of syngamous determination of sex is the fact than in bees the 

 fertilized ova develop into females, the unfertilized (parthenogenetic) ova into males. Other 

 experiments and observations have failed to throw much light upon any possible factors 

 such as the relative age and vigor of the parents or the relative vigor of the ova and sper- 

 matozoa in this type of determination. 



There is considerable experimental evidence in favor of epigamous determination. In 

 experiments where accurate observations have been made, it appears that the sex of the indi- 

 vidual depends, to some extent at least, upon nutrition. For example, a brood of caterpillars 

 was divided into two equal lots. In the period preceding the pupa stage, one lot was well fed, 

 the other lot poorly fed. The well fed lot produced sixty-eight females and four males; 

 the poorly fed lot produced seventy-six males and three females. In the case of plant lice, 

 observations show that during the summer when nutritive conditions are most favorable, 

 females only are produced; that in the autumn when the weather is colder and the food con- 

 ditions less favorable, both males and females are produced. In experiments on tadpoles the 

 percentage of females was from fifty-four to sixty-one in different broods of unfed animals; in 

 those fed with beef, seventy-eight; in those fed with fish, eighty-one; in those fed with frog 

 meat, ninety-two. 



Experiments on the higher animals are few, but it appears that the nutritional conditions 

 have some influence, although other factors of importance probably enter into the deter- 

 mination of sex. Giron divided 300 ewes into two equal groups. One group, which was 

 well fed and served by young rams, produced sixty per cent, of female offspring; the other 

 group, which was poorly fed and served by older rams, produced only forty per cent, of female 

 offspring. In the case of the human race it has been claimed that after a general depression in 

 nutritional conditions such as war, famine, pestilence, etc. there is an increased pro- 

 portion of male births. 



