118 CARNEGIE INSTITUTION OF WASHINGTON. 



to produce males are smaller, and have higher water-content and smaller 

 energj'-content, than those that produce females. This conclusion he 

 has repeated!}' confirmed at this Station. Especially has he been able, 

 by the use of the bomb calorimeter, to demonstrate that eggs destined 

 to become males contain less stored energy than eggs destined to 

 develop into females. Whether the difference in energy-content (how- 

 ever it may have arisen) is the cause of the difference in the eventual 

 sex or whether it is induced by a certain difference in the unfertilized 

 egg which determines the difference in storage metabolism is uncertain. 

 There is reason for thinking that the ova of birds are of two kinds, those 

 destined to produce males and those destined to produce females, and 

 there is also evidence that the former contain a sex chromosome which 

 the latter lacks. This difference in the chromosomal content of the 

 eggs destined to be males and females, respectively, may therefore be 

 the cause of the difference of energy-content of the two kinds of eggs. 

 This is a matter for further study. 



With the aim of determining whether a modification of the amount of 

 yolk stored in the egg can control the sex of the resultant chick, Dr. 

 Riddle has spent some time (partly in collaboration with Dr. Bassett) 

 in trying to induce such a modification. One result secured is that 

 when the female pigeon is subjected to alcohol vapor it lays smaller 

 eggs than normal. Other substances used (phloridzin and uro tropin) 

 have caused a reduction in the fertility of the egg, but have not 

 markedly altered its size. 



The foregoing studies are of interest because of their relation to a 

 normal determination of sex in the egg-laying of pigeons. If, as 

 Wliitman first pointed out, certain somewhat distantly related species 

 of pigeons be crossed and if the eggs be taken away as fast as laid, so as 

 to induce the pair to continue to lay fertile eggs, then in the beginning 

 of the season {i. e., in the spring) both eggs of a clutch will produce 

 nearly or quite exclusively males ; the last eggs laid in the autumn will 

 be nearly or quite exclusively females; while in the transition period 

 the first egg of the clutch usually produces a male and the second a 

 female. Of course, one can not say that the experiment has induced 

 any change in the sex-fate of the eggs ; it may possibly be that the dis- 

 tribution of the male and female eggs in the ovary is such that this 

 result necessarily follows from the forced heavy reproduction of the 

 mother. Dr. Riddle holds an alternative hypothesis, and an attempt 

 will be made to decide between them. 



Modification of Sex Behavior in Pigeons, Oscar Riddle. 



A very remarkable fact, in addition to those stated in the preceding 

 paragraph, is that when two full sisters from such series are hatched 

 from the two eggs of a single clutch the first hatched behaves in copu- 

 lation as though it were a male. Also females hatched early in the 

 season (the period when most males are produced) are more masculine 



