144 LOUISE B. WALLACE. 



like accessory chromosomes are more conspicuous than at any- 

 other time during the development of the sperm-cells (Fig. 13). 

 On the other hand, a study of the primary oocytes at a corre- 

 sponding stage reveals the fact that in them no trace of the 

 accessory chromosomes can be found (Fig. 57). It might be 

 contended that during the much longer resting stage of the 

 oocytes, the accessory chromosomes dissolve into the reticulum 

 as do the ordinary chromosomes but an argument against such 

 an interpretation is the fact that the same lack is evident even in 

 the extremely small ovaries of very young spiders captured in 

 June --two months before the breeding season. At all stages of 

 the growth period, in the youngest as well as in the oldest 

 oocytes, no accessory chromosomes can be found. Now, while 

 this is not in itself conclusive evidence that all of the cells of the 

 female spider lack the accessory chromosomes, it seems probable 

 that such is the case in the light of the work upon insects and 

 in view of the fact that one half of the spider spermatozoa lack 

 these two elements. Since the primary oocytes have no acces- 

 sory chromosomes, in all probability the mature eggs lack them 

 also. If, then, an egg be fertilized by a spermatozoon possessing 

 two accessory chromosomes, a male would be produced but if an 

 egg be fertilized by a spermatozoon which does not possess them, 

 a female would be produced. This interpretation brings the 

 spider into line with the insects in support of the view that the 

 accessory chromosomes may be directly connected with sex- 

 determination, the main difference between the insects and spiders 

 being that in the former the female has the greater number of 

 chromosomes while in the latter the male is the "favored" 

 one. 



spermatozoa ova 



~ 



