ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 363 



Rig-ht and Left Testes of Pig-eons. — Oscar Riddle {Anat. Record, 

 1918, 14, 283-334). In healthy adult doves and pigeons the right 

 testis is larger than the left in a very high percentage of cases. The 

 left testis, in a high percentage of cases, is absolutely longer and thinner 

 — more nearly the shape of the single persistent (left) ovary of the 

 female — than is the right testis. It is relatively longer and thinner in 

 probably nearly all cases. In disease — particularly in tuberculosis — the 

 testes undergo extreme atrophy (often DO to 95 p.c.) ; the reduction is 

 greater in the right than in the left testis ; the ovary probably suffers 

 no reduction whatever. In hybrids the normal size relations of the two 

 testes are much disturbed. It is suggested that a male which has been 

 forced to arise from a female-producing egg may show in the relative 

 size of its gonads an approximation to the relative size of the gonads of 

 a female. J. A. T. 



Influence of Age on Reproductivity of Fowls. — Raymond Pearl 

 (Genetics, 1917, 2, 417-32), A new constant, the reproductive or 

 fertility index, is proposed for the measurement of the net reproductive 

 ability of mated pairs of the domestic fowl. This index expresses the 

 actual number of chicks produced by the mating, and capable of living 

 three weeks after hatching, as a percentage of the maximum total 

 number of chicks physiologically possible. For a Barred Plymouth 

 Rock strain the reproductive index has a mean value of about 12 p.c. 

 Net fertility is a rather highly variable character, like other purely 

 physiological characters. Reproductive ability, as measured by the 

 index, diminishes with advancing age of the birds mated, having'' its 

 maximum when each of the birds mated is from ten to fourteen months 

 old. The rate of decline with advancing age is more rapid in the 

 males. J. A. T. 



Age and Fertility in Fowls. — Raymond Pearl (Froc. Nat. Acad. 

 Sci., 1917, 3, 354-56). In mammals it seems that fertility rises from 

 the beginning of sexual life to a maximum, and then declines with 

 further increase in age, until total sterility is reached. In the fowls 

 Pearl worked with there was no approach to this law of fertility. 

 Instead we find a steady and progressive decline in fertility after the 

 first breeding season. Up to a combined parental age of four years 

 what occurs is this : There is a significant drop in reproductive ability 

 as we pass from a combined age of two years for the mated birds to 

 three years. In passing from three years to four there is no significant 

 change in reproductive abihty. In passing from a combined age of 

 four years to that of five years there is a large drop in the net reproduc- 

 tive ability of the mating. J. A. T, 



Sex-ratio in Domestic Fowl. — Raymond Pearl [Science, 1917, 

 46, 220). The determination of sex in poultry is primarily a matter 

 of zygotic constitution, but under certain conditions the normal ratio 

 of about 48 '57 p.c. of cockerels may be modified. The chief factor 

 in bringing about the modification in the direction of a larger produc- 

 tion of pullets is the fecundity or laying ability of the hens used as 



