TWO KINDS OF GERM-CELLS 493 



in one half, and a " small idiochromosome " or Y-element in the 

 other half. 



(b) The evidence that the one set of spermatozoa induce 

 male-development and the other set female-development is 

 indirect ; it is obtained by an examination of the state of the 

 chromosomes in the body-cells of the offspring. The Y-element, 

 for instance, is found only in the males, while the X-element 

 is found in both sexes, but doubled in the female, single in 

 the male. 



(c) Wilson gives the following formulae: 



(a) In the absence of a Y-element. 



Egg X + spermatozoon X ~ zygote XX (female). 

 Egg X -f spermatozoon no X = zygote X (male). 



(&) In the presence of a Y-element 



Egg X + spermatozoon X = zygote XX (female). 

 Egg X + spermatozoon Y = zygote XY (male). 



In the German cockroach (Blatta germanica) and in the 

 " Red Bug " (Pyrrhocoris apterus), half of the spermatozoa 

 have the same number of chromosomes as the ripe ova (N), 

 including the accessory chromosome ; the other half have one 

 less (N i), being without the accessory chromosome. An 

 ovum with N, fertilised by a spermatozoon with N, results in 

 a fertilised ovum with aN, and this develops into a female. 

 An ovum with N, fertilised by a spermatozoon with N I, 

 results in a fertilised ovum with 2N I, and this develops 

 into a male. 



In the meal-worm (Tenebrio molitor) and in the house-fly 

 (Musca domestica) the number of chromosomes is the same 

 throughout, but in half of the spermatozoa one of the number 

 is small, and ova fertilised by these develop into males. 



A fine corroboration of the importance of the chromosomes 



