164 Hjfhrkls behveen Pheasant and Fowl 



In the Biston hybrids Harrison and Doncaster found that reciprocal 

 crosses gave very different results; thus from hirtaria female and zonaria 

 male both sexes were formed, but from the converse cross only males 

 developed. An explanation of this was nffered which assumed that in 

 the zonaria egg a certain anKnmt of male determining substance must 

 be present if fertilisation was to produce a male. The hirtarin sperm, 

 however, is supposed to possess a much greater ami mnt of this substance 

 than does the zonaria sperm, and thus from any zonaria egg which is 

 fertilised by a hirtaria sperm, male offspring will develop. This assumj)- 

 tion is supported by the fact that the hirtaria chromosomes are four 

 times as large as the zonaria ones, and if the male determining substance 

 is earned by a sex chromosome it will contain four times as much as 

 does any single zonaria sex chromosome. 



It is possible that the high male production in the hybrids I have 

 investigated is due to a similar cause, and that the pheasant sperm is 

 preponderately male producing when it fertilises the egg of the fowl. 

 No definite evidence of this has as yet been obtained, l)ut I liope to 

 conduct research on tiiese lines in tlic future. 



I wish to express my thanks to Mrs Haig Thomas and Mr Lewis 

 Jones for supplying me with material, and especially am I gratc^ful to 

 Dr Doncaster, who has read over this manuscript and offered mi' man}' 



\;iluablc suggestions. 



EXPLANATION OF PLATE IX. 



All the fisnres were drawn to the same scale. The outlines were drawn l>y means ot a 

 Zeiss-Greil drawing apparatus, and the details filled in free hand. 



Figures 1 — 10. Sperniatoyeuesis ot male pheasant. 



Fig. 1. Ecjuatorial plates of three spermatogonial cells. 



Fig. 2. Young primary spermatocyte. 



Fig. 3. Early synapsis stage : fine threads are withdrawn to one side of the nucleus. 



Fig. 4. Later synapsis stage with thick coiled threads. 



Fig. .5. Primary spermatocyte, eipiatoriiil jilate probahly with 10 chromosomes. 



Fig. G. Primary spermatocyte, equatorial plate probably with 11 chromusomes, one 



projecting beyond the others. 

 Figs. 7, 8. Division of primary spermatocytes: in Fig. 8 one of the chromosomes 



is differentiated from the others. 

 Figs. 9, 10. Secondary spermatocytes, equatorial plates. 



