THE FEMALE CHROMOSOME GROUPS IN 

 SYROMASTES AND PYRROCHORIS. 



EDMUND B. WILSON. 



The conditions seen in Syromastes marginatus L. are of inter- 

 est on account of the light that they throw on those observed by 

 Morgan in Phylloxera, as reported by him at the December meet- 

 ing of the American Society of Zoologists, and recently published 

 in the issue of Science for Feb. 5, 1909. In both forms the 

 " accessory " chromosome is not a single but a double body, and 

 the female chromosome-groups contain two more chromosomes 

 than the male. 



A reexamination of the spermatogenesis of Syromastes 1 led to 

 a confirmation of Gross's result that the spermatogonial number 

 is even (22), and that the " accessory " chromosome is formed by 

 the union of two chromosomes that are separate in the sperma- 

 togonia. This double element divides equationally in the first 

 spermatocyte-division but passes undivided to one pole in the 

 second, so that half the spermatozoa receive two more chromo- 

 somes (12) than the other half (10). This led me to the infer- 

 ence that the female somatic groups should have two more 

 chromosomes than the male i. e., 24 instead of 22, as had been 

 described by Gross. I had at that time no female material, but 

 through the kindness of Professor Boveri have since obtained an 

 abundant supply of the ovaries. Examination of this material 

 demonstrated the correctness of my earlier inference. A consid- 

 erable number of ovaries have been sectioned, many of which 

 contain numerous and very fine division-figures, showing the 

 chromosomes with great clearness. Whenever a good view of 

 the equatorial plate can be obtained, 24 chromosomes are unmis- 

 takably seen to be present, as is clearly shown in photographs. 

 Two figures (Fig. i, c, d} are appended, both of which were 

 drawn upon enlarged photographs by the method described in my 

 fourth " Study." Syromastes is an exceptionally favorable form 



1 " Studies on Chromosomes, IV., " Journ. E.vp. Zool., VI., i, 1909. 



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