Evolution and Adaptation

by Thomas Hunt Morgan

It has been claimed that the condition of nourishment of the mother may determine the number of eggs of a particular sex, but Cuénot found, in three species of flies which he raised, that there was a slight response in the opposite direction. He concludes that the condition of the mother is not a factor in the determination of sex.

The first egg of the two laid in each set by the pigeon is said, as a rule, to produce a male, and the second a female. Both Flourens and Cuénot found this to be the case in the few instances that they examined, but Cuénot has shown that this does not always happen. Even when this occurs, it has not been determined whether the result depends on something in the egg itself, that causes a male egg to be set free first, or on some external condition that determines that the first egg shall become a male. It has been claimed that the age of the spermatozoon might in this and in other cases determine the result; but Gerbe has shown that if the domestic hen is isolated for fifteen days after union with the male, she will continue to produce fertile eggs from which both sexes are produced, without showing any relation between the time the eggs are laid and the particular sex that develops.

Cuénot does not discuss whether sex is determined by the nucleus or by the protoplasm, but if, as he thinks probable, the size of the egg is a determining factor, it would appear that the protoplasm must be the chief agent. Even if this were the case it would still be possible that the size of the egg itself might be connected with some action on the part of the nucleus. If, as seems probable, identical twins come from halves of the same egg, then, since they are of the same sex, the absolute amount of protoplasm cannot be a factor in sex determination.

Fig. 6.—Diagram showing the maturation of the egg.

As a basis for the discussion that follows, certain processes that take place during the maturation divisions of the egg and of the spermatozoon must be briefly noticed. After the egg leaves the ovary it extrudes a minute body called the first polar body (Fig. [6 B, C, D]). This process of extrusion is really a cell division accompanied by the regular mitotic division of the nucleus; but since one of the products of the division, the polar body, is extremely small, the meaning of the process was not at first understood. The half of the nucleus, that remains in the egg, divides again, and one of its halves is thrown out into a second polar body (Fig. [6 E, F, G])). Meanwhile, the first polar body has divided into two equal parts, so that we find now three polar bodies and the egg (Fig. [6 G])). A strictly analogous process takes place in the formation of the spermatozoa (Fig. [7 B-F]). The mother-cell of the spermatozoon divides into two parts, which are equal in this case (Fig. [7 B-D]). Each of these then divides again (Fig. [7 E, F]), producing four cells that are comparable to the three polar bodies and the mature egg. Each of the four becomes a functional spermatozoon (Fig. [7 G, H]). Thus while in the maturation of the egg only the egg itself is capable of development, in the case of the male cells all four products of the two maturation divisions are functional.

Fig. 7.—Diagram showing the maturation of the spermatozoon.

Now, in certain cases of parthenogenesis, it has been found that one of the polar bodies may not be given off, but, remaining in the egg, its nucleus reunites with the egg nucleus, and thus takes the place of the spermatozoon, which does exactly the same thing when it fertilizes the egg, i.e. the nucleus of the spermatozoon unites with the nucleus of the egg. This fact in regard to the action of the polar body in fertilization is not as surprising as appears at first sight, for if each of the polar bodies is equivalent to a spermatozoon, the fertilization of the egg by one of its own polar bodies conforms to theory.

There is a considerable body of evidence showing that in many eggs at one of the two maturation divisions the chromatin rods derived from the nucleus are divided crosswise (Fig. [6 B, C]). The same thing occurs at one of the two divisions in the formation of the spermatozoon (Fig. [7 B, C]). At the other division to form the other polar body (or the other sperm-cell) the chromatin rods appear to be split lengthwise, as in ordinary cell division (Fig. [6 E, F, G]). In recent years the cross-division of the chromatin rods has attracted a great deal of notice, and Weismann in particular drew attention to the possible importance of this kind of division.