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