October 11, 1912] 



SCIENCE 



469 



sperm, differing in that one half the sperms 

 have a large chromosome and the other half 

 a smaller chromosome. Two such classes of 

 sperm were already known in certain other 

 insects, and McClung had earlier suggested 

 their connection with sex production. Miss 

 Stevens was among the first to establish the 

 correctness of this hypothesis by the discovery 

 that the small chromosome is confined to the 

 male line while in the female its place is 

 taken by the larger one. She drew the cor- 

 rect inference that since all unfertilized eggs 

 are alike in their chromosomal content, there- 

 fore a female results from the fertilization of 

 an egg by the sperm containing the larger 

 chromosome, and the male by the sperm con- 

 taining the smaller chromosome. A similar 

 relation was discovered at the same time by 

 Professor E. B. Wilson. Their joint dis- 

 covery marks the turning point in the history 

 of the theory of sex-determination. 



During the following six years Miss Ste- 

 vens extended her studies in this subject over 

 a wide field. In 50 species of beetle she 

 found an unpaired chromosome in twelve 

 cases, and an XY pair in thirty-eight cases, 

 and in nine species of flies she found an XY 

 pair of chromosomes. Such an extensive 

 study will not seem superfluous when the 

 reception of this important discovery in re- 

 gard to sex is remembered, for the profound 

 significance of the results were by no means 

 generally appreciated, and it is not going too 

 far to say that many cytologists assumed a 

 sceptical or even antagonistic attitude for 

 several years towards the new discovery. No 

 doubt this will be attributed to scientific cau- 

 tion, but conservatism may better account for 

 the slowness with which a recognition of this 

 discovery was received. It was said, for ex- 

 ample, that the unequal distribution of the 

 sex chromosomes is only an index of sofne 

 more profound changes taking place, and is 

 not in itself the real differential. In apparent 

 support of this objection was advanced the 

 fact — ^which Miss Stevens's work had also 

 helped to establish — that in a number of in- 

 sects the sex-chromosomes are equal in size. 

 The first objection is purely formal, for even 



if true the discovery would still remain of 

 prime importance as indicating when and how 

 an internal difference arises that leads to the 

 formation of the two sexes. In regard to the 

 fact appealed to in apparent support of this 

 objection, it has, more recently, become ap- 

 parent that the sex chromosomes are also re- 

 sponsible for a number of other differences, 

 in addition to that of sex determination. In 

 other words, sex determination is only one of 

 many " factors " carried by these chromo- 

 somes. If this is granted, the inequality in 

 size differences — one on which perhaps too 

 much emphasis was placed at first — is in itself 

 of no significance, although when such a dif- 

 ference is present it gives a clue to a funda- 

 mental relation which might otherwise escape 

 detection. The appeal, therefore, to the cases 

 where no difference in size can be detected, 

 has no significance, except in so far as an un- 

 fortunate emphasis laid on a size difference 

 gave the conservative-minded an opportunity 

 to insist on an unimportant criticism. 



Miss Stevens's first paper in 1901 gave a 

 complete account of the life cycle of the pro- 

 tozoan, Boveria, parasitic in Holothurians. 

 Later she discovered the occurrence of true 

 chromosomes in this form, and made out 

 many of the processes that take place during 

 conjugation. 



Four papers dealing with the chromosomes 

 in the life cycle of Aphids appeared in 1905, 

 1908, 1909 and 1910. The double number of 

 paired chromosomes was found in the par- 

 thenogenetic cycle, and the reduced number 

 in the sexual forms. The parthenogenetic 

 eggs were shown to give off a single polar 

 body; the sexual egg two polar bodies. Miss 

 Stevens denied at first the presence of an un- 

 paired sex chromosome in the spermatogenesis, 

 but later corrected this error. She failed to 

 note, at first, that the male had fewer chromo- 

 somes than the female, but later recognized 

 this difference. In her work on other insects 

 she described both an end-to-end union of 

 chromosomes, as well as a side-to-side pairing, 

 but her work on the synaptic stages was far 

 less complete and convincing than that on 

 other parts of the germ-cycle. At the time of 



