THE DETERMINATION OF SEX 573 



The evidence of Lhis is most complete in certain representa- 

 tives (now including nearly a hundred species) of the Hemiptera, 

 Coleoptera, Orthoptera, Diptera and Odonata ; but the same 

 principle no doubt applies to the myriapods and arachnids, 

 some of which are also known to produce two classes of 

 spermatozoa exactly corresponding to those of the insects. A 

 precisely similar relation has very recently been discovered 

 by Boveri and Gulick in the nematode Heterakis l ; and Boveri 

 believes that the facts reported by Boring in the classical 

 object, Ascaris megaloccphala 2 are open to the same interpre- 

 tation. On the other hand, it has recently been demonstrated 

 by Baltzer 3 that in certain sea-urchins the above relation is 

 in a general sense reversed, there being two kinds of eggs 

 and but one kind of spermatozoa ; in other words, in these 

 animals it is the female that is the heterogametic sex, while 

 the male is homogametic. It seems probable that the same 

 condition exists in Dinophilus apatris, where two kinds of 

 eggs have long been known. The experimental evidence 

 suggests that this is true also of certain other animals of 

 which cytological data are not yet at hand. As will be seen, 

 the results obtained on the sexual forms of insects are fully 

 confirmed by investigations on some of the parthenogenetic 

 species in which all the fertilised eggs produce females (aphids 

 and phylloxerans). 



The Cytological Facts 



In examining the cytological phenomena more in detail 

 (beginning with the insects) we have before us the following 

 two principal questions : 



(i) How .does the nuclear dimorphism of the spermatozoa 

 arise ? 



(2) What is the evidence that the two classes of spermatozoa 

 are respectively male-producing and female-producing? 



It is necessary at the outset to bear clearly in mind certain 

 familiar general relations of the chromosomes. In the sexually 

 produced organism the somatic cells and earlier generations 

 of germ-cells possess a double or diploid group of chromosomes. 

 At a certain period in the life-cycle the diploid number under- 

 goes reduction to one-half in those cells that are to produce 



1 Arch./. Zellforsch. 1909, 4, 1. 2 Ibid. 3 Ibid. 2. 



