Page:Popular Science Monthly Volume 85.djvu/136

132 found in two races of the thread worm, Ascaris megalocephala, 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 observed 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 spermatozoa; 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 indicate 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 R. 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 observation 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 vitality 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