ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 439 



gene, is an actual material unit of genetic segregation ; it is of unknown 

 nature, but probably consists of a genetically indivisible portion of a 

 chromosome (a locus) in a particular state. The preseuce-and-absence 

 scheme of factor notation properly employs only the first of these mean- 

 ings ; the Morgan-Castle scheme, on the other hand, may use either. 



Control of Sex-ratio.* — Oscar Riddle distinguishes two questions : 

 " Has a particular germ-cell which had initial tendencies to produce one 

 sex been experimentally forced to the production of the opposite sex ?" 

 and " Have the conditions of the experiment decreased or suppressed the 

 production, or hindered the union, or modified the chromosomal constitu- 

 tion of one of the types of germ-cell and left the other normal and 

 functional ?" These possibilities of accounting for abnormal sex-ratios 

 certainly exist, and they must be squarely met by decisive experiment. 



In some animals (e.g. insects and several mammals) one and the 

 same male produces spermatozoa of two kinds, which are not equal in 

 their prospective sex-value, fn other types (e.g. moths and birds) the 

 female produces two kinds of eggs, having opposite prospective sex 

 value. In parthenogenetic forms, such as bee, gall-fly, plant-louse, the 

 sex is known to bear certain relations to chromosome number or to 

 maturation phenomena in the egg. In wide crosses among Echino- 

 derms, Baltzer and Tennent have shown that when the cross is made in 

 one of the two possible directions, some of the chromosomes proceeding 

 from the sperms are eliminated and do not take part in embryo-formation. 

 This type of chromosome behaviour has been found, liowever, only in 

 crosses of very widely-separated forms. 



Pure wild species of doves and pigeons have proved to be almost 

 ideal material for obtaining highly abnormal sex-ratios, and for the 

 analysis of the significance of the modified ratios. Whitman showed 

 (1) that generic crosses, when not permitted to lay many eggs, produce 

 mostly or only males ; (2) that such pairs, when made to lay many eggs 

 (crowded reproduction), produce males predominantly from their earlier, 

 stronger eggs, and predominantly or only females from the later eggs 

 laid under stress of overwork ; and (3) that from eggs of pure wild 

 species the first egg of the pair or clutch more often hatches a male ; the 

 second egg of the pair more often produces a female. 



Riddle finds that the ova of the pigeon are certainly sexually di- 

 morphic. Nearly 900 individual yolks have been analysed, to determine 

 whether chemical differences are discoverable between male-producing 

 and female-producing ova. It was found (1) that the male-producing 

 egg of the spring is an egg that stores less material than does the 

 female-producing egg of the autumn ; (2) that the male -producing egg 

 of the clutch stores less material than does its female-producing mate ; 

 (3) that the eggs of old females store more material and yield a higher 

 percentage of females than those of birds not old. Thus it is evident 

 that the egg of female-producing tendency is one whose storage meta- 

 bolism is high, as compared with eggs of male-producing tendency. 

 Moreover, the analyses show that during the season successive clutches 



* Journ. Washington Acad. Sci.,vii. (1917) pp. 319-56. 



