160 Hi/hn'ds bettreen Pheasant and Fmd 



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 abudrnial 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. 



Discu.ssiox. 



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 sterde, partially so, or entii-ely 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 ime 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 

 wei-e 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 pa|)er two years later this observer and Mrs Haig Thomas 

 pid>lishi'd the ri'sults of expei-iments on the hybridisation of phea.sants. 



