BIRDS' EGGS 285 



Variational Sterility. — This term may be used for those 

 cases where the reproductive organs do not produce germ- 

 cells or efficient germ-cells. There is an inherent defect, 

 and it is often seen in hybrid offspring. Whether it is 

 something per se, or whether it should be ranked with other 

 germinal variations and mutations is a question too difficult 

 for our present knowledge. 



A female bird may be infertile in various degrees, as 

 A. Chapellier has shown (191 1) in regard to hybrids between 

 male goldfinch and female canary. The ovary may produce 

 no ova or very small ova ; the ovary may produce ova which 

 are not laid or which are non-fertilisable. Twenty different 

 hybrid females have been studied by H. Poll (191 1), who 

 finds that complete sterility in the female is marked by the 

 absence of small reserve follicles in the adult ovary and by 

 various signs of ovarian degeneration. There are hybrids, 

 however, which lay eggs and there are a few which lay eggs 

 that develop. 



In a study of the spermatogenesis of hybrids between 

 male wild pigeon and female domestic dove, Geoffrey Smith 

 found (191 2) that the ripe spermatozoa showed in certain 

 cases structural abnormalities, and were on an average twice 

 as large as the normal spermatozoa of either parental type. 

 All the spermatozoa were probably impotent ; these and 

 similar hybrids are invariably sterile. Inquiry showed that 

 the stumbling-block occurred during the maturation of the 

 sperm-cells, and was probably due, as Guyer suggested, to 

 the fact that the chromosomes derived from the specifically 

 distinct parents were incapable of harmonious fusion. 



A study of the sterility of hybrid pheasants by Geoffrey 

 Smith and Mrs. Haig Thomas (19 13) led to the conclusion 

 that it was connected with abnormalities occurring in the 

 process (synapsis stage) of the maturation of the germ-cells. 

 The spermatozoa of the hybrids are either deformed or 

 abnormally large. The immature ova fail to grow. Similarly, 

 in the case of male hybrid ducks {Cairina moschata and Anas 

 boschas), it has been shown by H. Poll (191 1) that up to a 

 certain point the spermatogenesis in the testes seems normal 



