556 MR. J. B. SUTTON ON ATAVISM. [DcC. 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 — 

 whetlier 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 hypo- 

 blast. 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 ? 

 , We know now 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 the 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 -^(pj of an inch, whereas the germinal 

 area of the ovum equals ^-i-^ 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, the 

 halves of a globe ^xTu of ^" i"'^h ^^ 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. 



We 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 m.anifesting themselves. 



When differentiation of sexes occurs in animals previously 

 hermaphrodite, it involves either the loss of certain characters on the 



