272 RECENT CYTOLOGY 



proceed to describe what actually seems to be an 

 example of this sort.* 



The case we have to describe is directly concerned 

 with one of the most interesting and elusive of bio- 

 logical problems namely, the problem of the heredity 

 of sex. Until recently opinion has been largely 

 dominated by the view that sex-production is in 

 general controlled by the influence of external con- 

 ditions. A large number of the earlier researches, 

 and some of the later ones, have, in fact, seemed to 

 show that sex is thus determined. The most recent 

 knowledge appears, however, to point conclusively to 

 the belief that sex is already determined in the fer- 

 tilized ovum. The fact that, so far as the evidence 

 goes, where more than one individual develops from 

 the same fertilized egg all are of the same sex seems 

 to point conclusively in this direction; and further 

 valuable evidence has recently been adduced. Professor 

 E. B. Wilson has recently investigated the behaviour 

 of the chromosomes in the somatic cells and in the 

 germ-cells of a particular species of insect known as 

 Protenor belfragi. The case afforded by this animal 

 is remarkable, inasmuch as the somatic cells in the 

 male, and only in the male, contain an odd number 



* [In the last few years definite evidence of the relation 

 between chromosomes as a whole and hereditary characters 

 has been obtained in several plants; e.g., the lata variety of 

 (Enothera is characterized by the possession of an extra 

 chromosome, and the variety gigas has twice the normal 

 number (Gates). Remarkable observations of a similar kind 

 have been made by R. P. Gregory in Primula sinensis (Proc. 

 Roy. Soc., B. 87, 1914, p. 484).] 



