178 PHYSICAL BASIS OF HEEEDITY 



her black color to only half of her grandsons. The chro- 

 mosomal explanation can obviously be worked out on the 

 same scheme as in Abraxas (Fig. 77). But if Guyer's 

 recent account of spennatogenesis in birds is correct, the 

 situation is different. Guyer describes the ripening of 

 the sperm as follows: There are 18 chromosomes in the 

 male, including two large Z's (16 + 2). After synapsis 

 there are 9 double chromosomes in the first spermatocyte, 

 all of which, except ZZ separate at the first maturation 

 division, 8 going to one pole and 8 to the other. One 



Barred c? Black p 



^' z^z^ zV 



zV zV 



1 Darredc? Barred 



z^z° z^z' zV zV 



Barredc? Barred c? Barredo Black 9 



FiQ. 77. — Scheme showing the transmission of the sex-Hnked characters B = barred, and 



b =black in the cross shown in Fig. 76. 



daughter cell gets both Z's {S + 2). This cell then divides 

 again, the Z's presumably separating so that two second 

 spermatocytes are produced, each with 9 chromosomes 

 (8+1), including the Z. These become the functional 

 sperm. The other spermatocyte, the one without a Z, 

 may divide again, but it, or its products, degenerate, and 

 never produce sperm. According to Guyer, there are 17 

 chromosomes in the female, including one Z. Presum- 

 ably, then, after reduction half of the eggs will contain a 

 Z (8 + 1), the other half will be without it (8). The egg 

 that carries a Z (8 + 1), fertilized by a sperm (each sperm 

 carries a Z (8 + 1)), will make a male with 18 chromo- 



