452 SUMMARY OF CURRENT RESEARCHES RELATING TO 



distribution curve indicating that the population of spermatozoa is made 

 up of two separate groups. A comparison of the degree of separation 

 of these modes with the expected degree as derived from a calculation 

 based on the chromosomal histories shows in general a striking corre- 

 spondence. The general conclusion is drawn that two size groups may 

 be distinguished in many of the species which show chromosomal differ- 

 ences in spermatogenesis, and that the size difference is based on the 

 difference in chromosomal content. If the hypothesis that the chromo- 

 somal differences are of sex-determining value is true, it follows that 

 the larger spermatozoa differ from the smaller ones in sex determina- 

 tion. While the method seems competent to yield a conclusive general 

 result, no attempt is made to minimize the many possible sources of 

 error liable to come into individual determinations. 



Inheritance of Fecundity.* — Raymond Pearl finds that there is 

 a marked difference in average egg-production per bird of Barred 

 Plymouth Rock pullets of the Maine Agricultural Station strain at the 

 present time as compared with what obtained during the period of 

 simple mass-selection for this character of fecundity. The difference 

 in question is in the direction of a substantially higher mean production 

 at the present time, when tested on flocks of large size. The increase in 

 flock average productivity is most pronounced in respect to the winter 

 production, the laying cycle to which especial attention has been given in 

 the breeding. The cause of this increase in flock productivity appears, 

 with a degree of probability which amounts nearly to certainty, to be 

 that the method of breeding the stock now followed is more closely in 

 accord with the mode of inheritance of fecundity than was the simple 

 mass-selection practised in the earlier period. The conclusion indicated 

 is that high fecundity is a sex-linked character, for which the female is 

 is heterozygous. This conclusion has also been reached by practical 

 poultrymen in their breeding operations. 



Sex-ratio in Pigeons.f — Leon J. Cole and W. F. Kirkpatrick 

 report the results of their experiments on sex-ratios in pigeons, together 

 with observations on the laying, incubation, and hatching of the eggs. 

 The birds used were mostly what are known as Long-faced Tumblers 

 (both Clean Legged and Muffed), but the stock was derived from various 

 sources, and included some Parlor Tumbler stock. The normal ratio of 

 the sexes of pigeons hatched is 105 males to 100 females. The death- 

 rate of squabs is especially high for the first two or three days after 

 hatching, and at about ten to fifteen days of age. When the two squabs 

 are of distinctly different size before the banding age (ten to fifteen days) 

 the larger squab is more often a male than a female. The death-rate for 

 the two sexes, in bisexual broods, is essentially equal. There is a higher 

 mortality of females in early adult ages, and this, along with the higher 

 proportion of males hatched, may result in an excess of males in adult 

 populations. The number of unisexual broods (" both males " or 

 " both females ") is very slightly greater than that of bisexual broods. 



* Amer. Nat., xlix. (1915) pp. 306-17. "1 



+ Bull. 162, Agric. Exper. Station, Rhode Island, 1915, pp. 463-510, 



