THE DETERMINATION OF SEX 95 



but the final result is the same, for there are formed two sexual 

 elements which have and two which have not an accessory 

 chromosome. Miss Stevens has investigated many insects, 

 as also has E. B. Wilson/^ Both have made similar dis- 

 coveries, and they have been able to demonstrate that the 

 accessory chromosome is not always single but may appear 

 in certain eggs as consisting of two, three, four, or even five, 

 parts. They have also observed in some species a second 

 accessory chromosome, which they have designated as the 

 Y-chromosome, and which perhaps also plays a role in the 

 determination of sex; but it must not be confused with the 



( ^V 



Fig. 42 a. Fig. 42B. 



Fig. 42. — Protenor belfragei. Chromosome groups. ^, from a cell of a female; 

 B, from a cell of a male. The accessory chromosomes are much larger than the ordi- 

 nary ones. 



Fig. 43. — Protenor belfragei. Second division of a spermatocyte. The large 

 accessory chromosome is moving undivided toward one pole. 



true accessory. Professor Wilson has had the kindness to 

 place at my disposition a number of photographs* of his 

 beautiful preparations, and from these Figs. 42-51 have been 

 sketched. In Fig. 42 the chromosomes are very distinct. In 

 Fig. 42 A, we can count very easily twelve ordinary chromo- 

 somes and two accessory. Fig. 42 B is similar. It also shows 

 twelve ordinary chromosomes, but only one accessory. The 



* During the lecture the original photographs were projected by the lantern. I 

 use this opportunity to express my very sincere thanks to Professor Wilson, both for 

 the loan of the photographs and for his generous permission to make drawings from 

 them. 



