i 3 2 THE POPULAR SCIENCE MONTHLY 



found in two races of the thread worm, Ascaris megalocepJiala, but it is 

 still too soon to affirm that this is true of white and black races of man, 

 though the facts seem to point in that direction. 



Similar correlations between chromosomes and sex have been ob- 

 served in more than one hundred species of animals belonging to widely 

 different phyla. In a few classes of animals, particularly echinoderms 

 and birds, the evidence while not entirely convincing seems to point to 

 the fact that two types of ova are produced and but one type of sperma- 

 tozoa; but the general principle that sex is determined by the chance 

 union of male-producing or female-producing gametes is not changed 

 by such cases. 



On the other hand, there are many observations which seem to indi- 

 cate that the sex ratio may be changed by environmental conditions 

 acting before or after fertilization and that therefore sex is determined 

 by extrinsic rather than by intrinsic causes. Most of these observations, 

 as already remarked, are now known to be erroneous or misleading, since 

 they do not prove what they were once supposed to demonstrate. But 

 there remain a few cases which can not at present be explained away in 

 this manner. Perhaps the best attested of these are the observations 

 of E. Hertwig and some of his pupils on the effects of the time of 

 fertilization on the determination of sex. If frog's eggs, which are 

 always fertilized after they are laid, are kept for some hours before 

 spermatozoa are mixed with them, or if the female is prevented for two 

 or three days from laying the eggs after they have entered the oviducts, 

 the proportion of males to females is enormously increased. Hertwig 

 attempts to explain this extremely interesting and important observa- 

 tion as due to the relative size of nucleus and cytoplasm of the egg; 

 but in general this nucleus-plasm ratio may vary greatly irrespective of 

 sex and there is no clear evidence that it is a cause of sex determination. 



Miss King, also working on frog's eggs, has increased the proportion 

 of males by slightly drying the eggs or by withdrawing water from them 

 by placing them in solutions of salts, acids, sugar, etc., but the manner 

 in which drying increases the proportion of males is wholly unknown. 



Extensive statistics show that in many animals including man more 

 males are born than females, whereas according to the chromosome 

 theory of sex determination as many female-producing sperm are formed 

 as male-producing. It is possible to explain such departure from the 

 1 : 1 ratio of males and females in conformity with the chromosome 

 theory if one class of spermatozoa are more active or have greater vital- 

 ity than the other class, or if after fertilization one sex is more likely 

 to live than the other. In the human species it is known that mortality 

 is greater in male babies before and after birth than in female babies, 

 and if before fertilization the activity or vitality of male-producing 

 spermatozoa is greater than that of female-producing ones it would ex- 



