286 THE BIOLOGY OF BIRDS 



enough. But they break down when it comes to forming 

 functional spermatozoa. If these defects in the sex-cells 

 be traced a little further back they may be referred to 

 something incompatible in the chromosome equipment of 

 the parental sex-cells. In some mammals which yield 

 sterile offspring when crossed the numbers of the chromo- 

 somes in the two parents are too different to admit of the 

 establishment of a successful Hneage of germ-cells in the 

 offspring. But there may be other forms of incompatibility 

 besides too great numerical difference between the chromo- 

 some groups of the two parents. 



Pointing in the same direction are the observations of 

 Professor M. F. Guyer on the male organs of hybrids 

 between cock and guinea-fowl. The testes were well- 

 developed, but no spermatozoa were formed. As in hybrid 

 pigeons there is a critical phase — the synaptic phase — which 

 few of the developing germ-cells (spermatocytes) can get 

 past, the chromosomes of the two parents being seemingly 

 unable to unite normally. 



Gametic Sterility. — In a successful cross between birds 

 of different varieties or species, the fertilised egg develops 

 into an able-bodied offspring, but this hybrid may be 

 innately sterile. It may fail to produce any germ-cells at 

 all, or its germ-cells may be non- viable. But there are 

 attempted crosses which do not result in offspring. The 

 ova will not be fertilised by the spermatozoa, or the sperma- 

 tozoa will not fertilise the ova. This is incompatibility of 

 germ- cells which are normal and vigorous. 



The general conclusion hinted at from the facts known in 

 regard to sterility in birds is that it concerns the bearers of the 

 hereditary traits — the chromosomes. Max Morse (1910) 

 gives a cautious expression of this view in the sentence : 

 " All of the studies which have been made point to the con- 

 clusion that whatever be its nature, there is an ' incom- 

 patibility ' existing between the chromosomes of individuals 

 of different species or varieties." 



