162 F. B. SUMNER, M. E. MCDANIEL AND R. R. HUESTIS. 



Pearl and Pearl (1908). Some of these differences \ve have 

 shown to be much more highly " significant " according to ac- 

 cepted standards than the differences between the pure and 

 hybrid races with which these authors were concerned. 



I )|SCI'SSION. 



In a field as well tilled as that of sex determination, it is both 

 impracticable and undesirable for us to enter into any extensive 

 review of the literature, and we offer no apology for the inade- 

 quate references to other workers which we have found it possible 

 to include in the foregoing pages. Those who are unfamiliar 

 with the literature of this field are referred to works by Geddes 

 and Thomson (1014). Marshall (1910), Morgan (1913), and 

 Horn-aster (1914). The last two of these books are of particular 

 value in this connection and supplement one another very nicely, 

 since the viewpoint of the authors differs rather widely. 



In recent vears the view has been gaining ground that the 

 sex of the individual is determined by the chromatin constitution 

 of one or the other of the germ cells. In the majority of cases 

 which have been investigated, it is believed that the differential 

 factor is contained in the sperm cell, though there is evidence in 

 a number of instances that the ovum is the gamete concerned. 

 The cytological and genetic data upon which these conclusions are 

 based are too well known to be discussed here. For many organ- 

 isms tin- evidence is doubtless wholly convincing. 



Many well-attested cases exist, however, in which the sex ratio 

 is known to be correlated with certain external factors. Those 

 who wish to universalize the chromosomal theory of sex deter- 

 mination, dispose of such cases by assuming the existence of either 

 a differential mortality among the developing organisms, or a 

 differential mortality among the sex-determining gametes, or some 

 other factor favoring one class of gametes or zygotes, as com- 

 pare. 1 with the other. It is assumed by such biologists that the 

 sex f the organism is invariably predetermined in the nucleus of 

 the fertili/ed egg, and is not subject to reversal by any influence 

 that can be brought to bear subsequently. 1 



i Morgan (1913), for example, devotes a section of a chapter to "The 

 Abandoned View that External Conditions Determine Sex," though there are 

 reasons for believing that Professor Morgan does not take such an extreme 

 position at the present time. 



