368 INTRODUCTION TO CYTOLOGY 



Mendelian theory of sex, constant deviations from this ratio are fre- 

 quently found. Still more significant is the fact that in many cases the 

 ratio can be markedly altered by changing the environmental condi- 

 tions. Thus R. Hertwig (1906, 1912) and Kuschakewitsch (1910) found 

 that if the eggs of the frog are allowed to become over-ripe before 

 fertilization, in which case they take up an abnormal amount of water, 

 the resulting individuals show an unusually high percentage (even 100 

 per cent) of males. Conversely, Miss King (1907-1912) lowered the 

 water content of toad eggs, and with a mortality of only a little over 6 

 per cent obtained 80 per cent females.- 



In Dinophilus, as already noted, there are two kinds of eggs laid: 

 large ones developing into females and small ones developing into males. 

 Malsen (1906) fouDcl that by altering the temperature the relative pro- 

 portion of the two sexes could be changed, but this effect was brought 

 about through an influence on the laying of the eggs: both kinds were 

 produced as usual, but the laying of one kind was hindered. 



The rotifer, Hydatina senta (Whitney 1914, 1916), if scantily fed on 

 Polytoma, continues to produce generations of parthenogenetic females, 

 but when copiously fed on Euglena females appear which lay male-pro- 

 ducing eggs, and sexual reproduction then occurs. According to Shull 

 and LadofT (1916) the percentage of males is here correlated with the 

 supply of oxygen which counteracts certain agencies (accumulated 

 substances in the water) tending to decrease male production. Whitney 

 (1919), however, contends that oxygen is not a factor affecting sex in 

 Hydatina. 



In interpreting such results as these considerable care should be 

 exercised in distinguishing an actual determination of the sex of an 

 individual from a number of other phenomena which, though they may 

 appear like sex-determination, are not to be regarded as such in the 

 strict sense. In many cases in which changed environmental factors 

 have been shown to have an influence on the sex ratio it is clear that the 

 results are not due to an actual reversal or determination of the sex in 

 any individual, but rather to the fact that the new conditions imposed 

 have caused a greater mortality among the eggs or embryos of one sex, so 

 that those of the other sex preponderate. Although the ratio of the sexes 

 may here be subject to an experimental control, the sex of no given 

 individual is actually determined or altered. 



Indirect control of another type is seen in organisms whose sex is 

 dependent upon the form of reproduction (zygogenetic or partheno- 

 genetic). Environmental conditions may in such cases influence the form 

 of reproduction resorted to, and therefore the sex of the animals resulting; 

 but the sex of no individual, once started, is altered. Morgan points out 

 that the change of diet in Hydatina, instead of altering sex directly, 

 induces the formation of a new type of female which may either function 



