160 Hybrids between Pheasant and Fowl 



form a varying number of irregularly shaped clumps of chromatin. 

 There is no uniformity as regards the number produced and a series can 

 easily be formed in which the number varies from two to three large 

 masses (Fig. 20), up to the condition where the chromosomes are almost 

 all bivalent, or where many have separated as univalents (Fig. 21). 

 This failure of the synaptic threads to form bivalent chromosomes is 

 evidently the cause of the sterility in the hybrids. The conditions are 

 •seen in Figs. 20 to 27. 



This is the final stage in spermatogenesis, for divisions of the primary 

 spermatocytes do not occur, nor is there any trace of abnormal mitoses. 

 The formation of giant spermatids recorded for hybrid pigeons by 

 Geoffrey Smith does not take place, nor is there any evidence of the 

 production of multiple spindles, as found by Guyer in hybrid pigeons 

 and by Wodsedalek in the testes of the mule. 



Discussion. 



As was pointed out at the beginning of the paper, our knowledge of 

 the cytology of the gonads of hybrids is still small ; the results, however, 

 of investigations which have been made, are extremely interesting as 

 regards the behaviour of the chromosomes during the maturation of the 

 germ cells. In both animals and plants the offspring of a cross may be 

 completely sterile, partially so, or entirely fertile, and the evidence which 

 we have points to the conclusion that one or the other of the results is 

 largely dependent upon whether the paternal and maternal germ plasms 

 are able to co-operate one with another during maturation. 



Guyer concluded, from investigations on hybrid pigeons, that there 

 was something repellent in the two germ plasms, which had contributed 

 to the cross, so that the chromosomes were unable to pair, causing an 

 irregularity in synapsis. 



Geoffrey Smith obtained three sterile male birds from the cross 

 domestic dove female with Magpie pigeon male. Synapsis was disturbed, 

 but not to the same extent as with Guyer 's birds, for abnormal spermatozoa 

 were produced. The reductive divisions, however, were abnormal for 

 no bivalent chromosomes were produced. The second division was 

 entirely suppressed and the spermatocytes proceeded without further 

 development to form giant spermatids from which spermatozoa of ab- 

 normal size developed. 



In a further paper two years later this observer and Mrs Haig Thomas 

 published the results of experiments on the hybridisation of pheasants. 



