TESTES OF GUINEA-CHICKEN HYBRIDS 55 



germ-plasms so dissimilar." The discovery of an X-element, how- 

 ever, together with the knowledge that it is of maternal instead of 

 paternal origin possibly gives us a simpler and more plausible 

 explanation. It is the spermatozoon without the large X-element 

 which unites with the egg in the production of the new male, and 

 since such a spermatozoon is much smaller than one of the other 

 type, the whole question may resolve itself into a mere matter 

 of the relative sizes of the spermatozoa. For inasmuch as such 

 hybrids are obtained with difficulty even under the most favor- 

 able conditions we may reasonably suppose that the egg-plasm 

 is more or less resistant or antagonistic to the entrance of a foreign 

 sperm, and that because of this the smaller type of spermatozoon 

 enters more readily, with the result that a male is produced. 



SUMMARY 



1. With one exception, where one testis was greatly hyper- 

 trophied, the testes of the four guinea-chicken hybrids examined 

 were of normal size. 



2. Microscopic examination showed them to be abnormal, 

 however. No spermatozoa were developed and the seminiferous 

 tubules were few in number in most regions of the testis and often 

 contained disintegrating and defective cells. 



3. As in hybrid pigeons the critical point seemed to be the 

 synaptic phase, the chromosomes of different parentage seemingly 

 being unable to unite normally in many instances. 



4. In spite of this difficulty, however, not a few first sperma- 

 tocytes succeeded in passing through synapsis and subsequent 

 division with more or less of an appearance of normality. 



5. An accessory chromosome or X-element of the guinea 

 (maternal species) type is present. 



6. The X-element is of large size in the common fowl and con- 

 sequently the mature spermatozoa without it are much smaller 

 than the ones which bear it. It is suggested that inasmuch as 

 males are produced only from eggs fertilized by a spermatozoon 

 without the X-element, the great preponderance of males among 

 such hybrid offspring may be due to the simple fact that the 

 smaller type of spermatozoon can more readily penetrate a foreign, 

 and hence more or less incompatible, egg-plasm. 



