ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 53 



the organs in development. Thus the last organs to differentiate in 

 the pupa and the first to be intersexual are the branching of the 

 antennae and the coloration of the wings. The first imaginal organ 

 differentiated in the caterpillar, and the last in the series to be changed 

 toward the other sex, is the sex-gland. 



The theory that best fits the facts is that the sex-factors are enzymes 

 (or bodies with the properties of enzymes) which accelerate a reaction 

 according to their concentration. In the fertilized egg the enzymes 

 which govern the differentiation of the organism towards one of the two 

 alternatives, maleness and feninleness, are both present. They may be 

 called gynase and andrase. The mechanism of sex-distribution — i.e. 

 through the sex-chromosomes — results in the formation of two kinds 

 of fertilized ova, differing in the relative concentration of the two 

 enzymes. Higher concentration results in greater rapidity of reaction, 

 and the more rapid reaction wins. The dominating enzyme, present 

 in higher concentration, will first succeed in furnishing the necessary 

 amount of specific substance acting as determiner, which may be called 

 the hormone of male or female differentiation. In intersexual forms, 

 which result in conditions of abnormal concentrations of enzymes, 

 development must go on under the influence of one enzyme up to a 

 certain point and then continue under the influence of the other. " A 

 given organ develops, in the case of female intersexuality, on female 

 lines up to a given point, when suddenly the male stimulus starts, 

 and the rest of the development is purely male. The degree of inter- 

 sexuality is determined by how long the development has been in progress 

 before the turning-point occurs." 



As regards the cytological aspect of the case, Goldschmidt makes 

 the following suggestion. The chromosomes cannot be regarded as 

 built up from chromatin particles, which are themselves the chemical 

 basis of heredity. The chromatin is rather a skeletal substance which 

 works as an " adsorbens " for the enzymes, which really constitute the 

 chemical basis of heredity. The quantitative behaviour of the enzymes 

 is of fundamental importance for the process of heredity. " The 

 quantity of adsorption of an enzyme by an adsorbens depends upon 

 the qualities of both and the surface of the adsorbens. Th^ wonderful 

 uniformity of size and shape of the chromosomes of a given animal 

 appears, therefore, as a minute mechanism to guarantee the typical 

 quantity of enzymes of heredity to be assembled at the moment of 

 fertilization. And all the strange processes preceding the maturation 

 of the sex-cells appear easily understandable, as well as the meaning 

 of the peculiar mechanism of mitosis. The formation of a chromosome 

 means, physically, the same thing as the dropping of a piece of charcoal 

 into a solution containing enzymes." 



Bristle Inheritance in Drosophila.* — Edwin Carleton MacDowell 

 has worked with a race of flies with extra bristles. Selection was con- 

 tinued for forty-nine generations for the production of high numbers of 

 extra bristles. In any generation after the early ones the distribution 

 of a single family is similar to that of the distribution of all the families 



* Journ. Exper. Zool., xxiii. (1917) pp. 109-^6 (10 figs.). 



