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Show 556 MR. J. B. SUTTON ON ATAVISM. [Dec. 21, secondary sexual characters can be explained on the principle known as the " Correlation of Organs." In order, however, to render this explanation tenable, it will be absolutely necessary to prove, as far as possible, that the germs of secondary sexual organs, which only manifest themselves occasionally in the females of dimorphic forms, are really inherited ; and, if the female is furnished with the germs of these structures, to show by what method they are transmitted. It is to this somewhat intricate part of the question that we must now direct our attention. If a careful analysis be made of those structures which constitute secondary sexual characters, we shall find that they are almost entirely developed in connection with the integument. The majority- whether they be horns, bristles, spurs, or teeth, for attack or defence ; or exuberance of hair, feathers, wattles, combs, &c, for aesthetic purposes,-the integument and the immediately subjacent tissues are responsible for them. In the earliest embryos of most mammals we distinguish at a very early stage two layers of cells, known as the epiblast and hypoblast. In most of the Metazoa a new layer is interposed known as the mesoblast; this originates in part from the epi- and in part from the hypoblast. It is the epiblast and the portion of mesoblast immediately adjacent that furnishes secondary sexual organs. From whence is the epiblast derived 1 .-.-, W e know new that the essential act of impregnation consists in the union of a spermatozoon with an ovum ; the head of the spermatozoon constitutes the male pronucleus, the germinal area the female pronucleus. After the pronuclei have united segmentation begins, and the formation of the fundamental layers is quickly brought about. There are good grounds for believing that tbe initial streak indicating the commencement of segmentation really marks the line of fusion between male and female pronuclei. If we measure the size of the head of a functional spermatozoon, it will be found equal to ^nnj of an inch, whereas the germinal area of the ovum equals 3-^3- of an inch. It is a very significant fact that the segments which give rise to the epiblast are smaller than those which furnish the hypoblast and mesoblast; to this there are very few exceptions. This is exactly what might be expected, tbe halves of a globe ^^V,j of an inch in diameter would certainly be smaller than the halves of a sphere one twelfth the size. The facts at our disposal seem to point to the conclusion that the epiblast is chiefly derived from the male element, while the female pronucleus is responsible chiefly for the hypo- and greater portion of the mesoblast. If this be true, the transmission of characters peculiar to the male is not so obscure as many have supposed. W e must now inquire how it is, that if the female possesses all the secondary sexual characters of the male in a latent manner, what is it that prevents them manifesting themselves. When differentiation of sexes occurs in animals previously hermaphrodite, it involves either the loss of certain characters on the |
