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When the protoplasm containing these granules was killed with a hot needle, development in some cases took place and an embryo was formed, but the embryo contained no germ cells. Here no injury had been done to the zygote nucleus, but these particular granules and the portion of protoplasm containing them were necessary for the formation of germ cells.
But if the zygote, although compounded of ovum and sperm, is predestined to give rise in the gametes descended from it, either to sperms only or to ova only, it may be suggested that all the somatic cells descended from the zygote are likewise either male or female, although they do not give rise to gametes.
But castration shows that the antlers of stags and other such characters are not determined in the zygote when the sex is determined, but owe their development, partly at least, to the influence of another part of the body, namely, the testes during the subsequent life of the individual.
When one difference between two breeds is considered, e.g. rose comb and single in fowls, and individuals are crossed, we have the determinant for rose and the determinant for single in the same zygote. The result is that rose develops and single is not apparent. In the next generation rose and single appear, as at the beginning, in separate individuals.
This theory would be quite compatible with the belief that adaptive somatic sex-characters may be due to external stimulation, for supposing that the hypertrophy or modification is conveyed to the determinants in the gametocytes, and was confined to one sex, e.g. the male, then these determinants would be modified in association with the sex-chromosomes of that sex, and thus though after reduction and fertilisation they would be present in the female zygote also, they would not develop in that sex.
What meaning are we to attach to the words 'male ovum' or even 'male producing ovum'? It is a fundamental principle of Mendelism that the soma does not influence the gametocytes or gametes; we have therefore only to consider the sex of the gametes themselves, derived from a zygote which is formed by the union of two sexes.
The zygote has the power of developing particular structures and characters in the complicated organisation of the adult, and we recognise that the characters are determined by the properties and constitution of the zygote; that is to say, of one or both of the gametes which unite to form the zygote.
Every zygote must have the same composition, and yet give rise to two sexes in the same individual. Further light on the sex problem, as in many other problems in biology, can only be obtained by more knowledge of the physical and chemical processes which take place in the chromosomes and in the relations of these structures to the rest of the cell.
and the character M would only appear in the male because it only develops in association with XX in the somatic cells descended from the male zygote. This would be the result in the first generation in which a somatic modification affected the factors in the chromosomes.
In parthenogenetic ova this happens without conjugation with a spermatozoon at all: in other cases, since the zygote is compounded of spermatozoon and ovum, we can only say that in the XX zygote, the ovum developing only ova, the female is dominant, in the X or XY zygote developing only spermatozoa the male is dominant.
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