Evolution and Adaptation

by Thomas Hunt Morgan

There is another fact that gives this division especial significance. It has been discovered that the number of chromosomes that appears in each dividing cell of the organism is a constant number, but it has also been discovered that the egg, before extruding its polar bodies, and the mother-cell of the spermatozoon (Figs. [6], [7 B]), contain exactly half of the number of chromosomes that are characteristic of the body-cells of the same animal (Figs. [6], [7 A]). Now there is good evidence to show that the reduction in number is due to the chromosomes uniting sometimes end to end in pairs, as shown in Figures A and B. Furthermore, it has been suggested that at one of the maturation divisions, when the chromosomes divide crosswise, the united chromosomes are separated (Figs. [6], [7 B, C]), so that one remains in the egg and the other goes out into the polar body. The same thing is supposed to occur at one of the maturation divisions of the sperm mother-cell. A further consideration of capital importance in this connection has been advocated by Montgomery and by Sutton, namely, that, when the chromosomes unite in pairs, a chromosome from one parent unites with one from the other parent. Consequently at one of the two reduction divisions maternal and paternal chromosomes may separate again, some to go to one cell, some to the other.

When the spermatozoon enters the egg it brings into the egg as many new chromosomes as the egg itself possesses at this time, and the two nuclei, uniting into a single one, furnish the total number of chromosomes characteristic of the animal that develops from the egg. At first the chromosomes that are brought in by the spermatozoon lie at one side of the fused nucleus, and those from the egg itself at the other side. This arrangement appears, however, in some cases at least, to be lost later. At every division of the nucleus, each chromosome divides and sends a half to each of the daughter-nuclei. Thus every cell in the body contains as many paternal as maternal chromosomes. This statement also applies to the first cells that go into the reproductive organs, some of which become the mother-cells of the germ-cells. Later, however, in the history of the germ-cells,—just before the maturation divisions,—these chromosomes are supposed to unite in pairs, end to end, as explained above, to give the reduced number. Later there follows the separation of these paired chromosomes at one of the two maturation divisions. If at this time all the paternal chromosomes should pass to one pole, and all the maternal to the other, the germ-cell ceases to be mixed, and becomes purely paternal or maternal. If this ever occurs, the problem of heredity may become simplified, and even the question of sex may be indirectly involved; but it has not been established that, when the reduced number of chromosomes is formed, there is a strict union between the paternal and maternal chromosomes, and if not, the subsequent separation is probably not along these lines. If, however, the chromosomes contain different qualities, as Boveri believes, there may be two kinds of eggs, and two kinds of spermatozoa in regard to each particular character. It is this last assumption only that is made in Mendel’s theory of the purity of the germ-cells.

Several attempts have been made at different times to connect the facts in regard to the extrusion of the polar bodies with those involved in the determination of sex. Minot suggested, in 1877, that the egg ejects by means of the polar bodies its male elements, which are again received in the fertilization of the egg by the spermatozoon. The same idea has also been expressed by others. It has been objected to this view that one polar body ought to suffice, and that no similar throwing out of part of its substance is found in the process of formation of the spermatozoon, which should, on the hypothesis, throw out its female elements. It would seem, on first thought, that this view might find support in the idea expressed above, namely, that in one of the polar bodies half of the chromosomes pass out, so that there is conceivably a separation of the maternal from the paternal. If this were the case also in the spermatozoa, then two of each four would be paternal and two maternal. This is, however, a very different thing from supposing them to be male and female, for it by no means follows, because the chromosomes correspond to those of the father or of the mother in the sum of their characters, that they are, therefore, also male or female in regard to sex.

It has been pointed out already, that in most parthenogenetic eggs only one polar body is extruded. There are, it is true, a few apparent exceptions to this rule, but in most cases it is certain that only one is extruded. In several cases the beginning of the formation of the second maturation division of the nucleus takes place, but after the chromosomes have divided they come together again in the nucleus. If each polar body be interpreted as equivalent to a spermatozoon, then this result is rather a process of self-fertilization than true parthenogenesis. It is, nevertheless, true that in some cases development seems to go on after both polar bodies have been extruded. Moreover, it has been found possible to cause the eggs of the sea-urchin to begin their development by artificial solutions after they have extruded both polar bodies. A single spermatozoon may also produce an embryo if it enters a piece of egg-protoplasm without a nucleus. The last instance is a case of male parthenogenesis, and if the theory of the equivalency of spermatozoon and egg be correct, this is what should occur.

Quite recently, Cuénot, Beard, Castle, and Lenhossek have contended that the differentiation of sex is the outcome of internal factors. They think that the view that sex is determined by external agents is fundamentally erroneous. The fallacies that have given rise to this conception, Castle points out, are, first, that in animals that reproduce sometimes by parthenogenesis and sometimes by fertilized eggs, the former process is favored by good nutrition and the latter by poor nutrition. This only means, in reality, Castle thinks, that parthenogenetic reproduction is favored by external conditions, and this kind of reproduction, he thinks, is a thing sui generis, and not to be compared to the formation of more females in the sexual forms of reproduction. There is no proof, however, that this is anything more than a superficial distinction, and it ignores the fact that in ordinary cases the females sometimes lay parthenogenetic eggs which differ, as far as we can see, from eggs that are destined to be fertilized in no important respect. More significant, it seems to me, is the fact that only parthenogenetic females develop the following spring from the fertilized eggs of the last generation of the autumn series, whose origin is described to be due to lack of food. We find, in the case of aphids, that unfertilized parthenogenetic eggs and also fertilized eggs give rise to females only, while a change in the amount of food causes the parthenogenetic eggs to give rise both to males and to females. This point is not, I think, fully met by Castle, for even if the change in food does not, as he claims, cause only one sex to appear, yet lack of food does seem to account for the appearance of the males at least.

The other fallacy, mentioned by Cuénot, is that the excess of males that has been observed when the food supply is limited is due to the early death of a larger percentage of females, which require more food, but this still fails to account for the excess of females when more food is given, provided Yung’s experiments on tadpoles are correct. It may be, however, in the light of Pflüger’s results, that there has been some mistake in the experiments themselves.

We may now proceed to examine Castle’s argument, attempting to show in what way sex is predetermined in the embryo. His hypothesis rests on the three following premises: “(1) the idea of Darwin, that in animals and plants of either sex the characters of the opposite sex are latent; (2) the idea of Mendel, that in the formation of the gametes [germ-cells] of hybrids a segregation of the parental characters takes place, and when in fertilization different segregated characters meet, one will dominate, the other become latent or recessive; (3) the idea of Weismann, that in the maturation of egg and spermatozoon a segregation is attended by a visible reduction in the number of chromosomes in the germinal nuclei.”

Expressed in a somewhat more general way, Castle suggests that each egg and each spermatozoon is either a male or a female germ-cell (and not a mixture of the two), and when a female egg is fertilized by a male spermatozoon, or vice versa, the individual is a sexual hybrid with one sex dominating and the other latent. The assumption that there are two kinds of eggs, male and female, and two kinds of spermatozoa, male and female, is not supported by any direct or experimental evidence. Moreover, in order to carry out the hypothesis, it is necessary to make the further assumption that a female egg can only be fertilized by a male spermatozoon, and a male egg by a female spermatozoon. While such a view is contrary to all our previous ideas, yet it must be admitted that there are no facts which disprove directly that such a selection on the part of the germ-cells takes place. If these two suppositions be granted, then Castle’s hypothesis is as follows:—

In order that half of the individuals shall become males and half females it is necessary to assume that in some individuals the male element dominates and in others the female, and since each fertilized egg contains both male and female elements, it is necessary to assume that either the egg or the spermatozoon contains the dominating element.

Castle supposes that in hermaphroditic organisms the two characters “exist in the balanced relationship in which they were received from the parents,” but, as has just been stated, in unisexual forms one or the other sex dominates, except of course in those rare cases, as in the bees and ants, where half of the body may bear the characters of one sex, and the other half that of the other sex.