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 + spermatozoon no X = zygote X (male). 



(b) 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 " [Pyrrhocovis 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 — 1), 

 being without the accessory chromosome. An ovum with N 

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

 with 2N, and this develops into a female. An ovum with N 

 fertilised by a spermatozoon with N — 1 results in a fertilised 

 ovum with 2N — 1, 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 

 has been recently afforded by the work of T. H. Morgan on 

 Phylloxera and of von Baehr on Aphis saliceii. In these forms 

 half of the spermatocytes degenerate (as Meves pointed out in the 

 bee), namely those without the accessory chromosome ; there- 

 fore all the spermatozoa are female-producers, and every one 



