Parthenogenesis (development of eggs without agency of male sperm) proves that in many simple forms the female nucleus alone possesses all the essential determiners for a new individual.  Boveri’s classic experiment[10] proved the same thing for the male nucleus.  He removed the nuclei from sea-urchin eggs and replaced them with male nuclei.  Normal individuals developed.  To make things still more certain, he replaced the female nucleus with a male one from a different variety of sea-urchin.  The resulting individual exhibited the characteristics of the male nucleus only—­none of those of the species represented by the egg.  Here, then, was inheritance definitely traced to the nucleus.  If this nucleus is a male the characters are those of the male line; if a female those of the female line, and in sexual reproduction where the two are fused, half and half.

Yet the fact remained that all efforts to develop the spermatozoon alone (without the agency of any egg material at all) into an individual had signally failed.  Conklin[11] had found out in 1904 and 1905 that the egg cytoplasm in Ascidians is not only composed of different materials, but that these give rise to definite structures in the embryo later on.  So a good many biologists believed, and still believe[12,13,14] that the egg is, before fertilization, a sort of “rough preformation of the future embryo” and that the Mendelian factors in the nuclei “only impress the individual (and variety) characters upon this rough block.”

If we look at these views from one angle, the apparent conflict disappears, as Professor Conklin[15] points out.  We can still presume that all the factors of inheritance are carried in the nucleus.  But instead of commencing the life history of the individual at fertilization, we must date it back to the beginning of the development of the egg in the ovary.  Whatever rude characters the egg possesses at the time of fertilization were developed under the influence of the nucleus, which in turn got them half and half from its male and female parents.  These characters carried by the female across one generation are so rudimentary that they are completely covered up, in the developing embryo, by those of the new nucleus formed by the union of the sperm with the egg in fertilization.

In case fertilization does not take place, this rude beginning in the egg is lost.  Since no characteristic sex is assumed until after fertilization, we may say that life begins as neuter in the individual, as it is presumed to have done in the world.  It will occur to those inclined to speculation or philosophic analysis that by the word “neuter” we may mean any one or all of three things:  (a) neither male nor female; (b) both male and female, as yet undifferentiated, or (c) potentially either male or female.  Clearly, the above explanation assumes a certain germinal specialization of the female to reproduction, in addition to the body specialization for the intra-parental environment (in mammals).