? McCLUNG. 



of the accessory chromosome in various species, enough evidence 

 is forthcoming to show wide departures from anything that might 

 be considered a typical form. And here it is that it may be 

 possible to secure more or less definite information with regard 

 to the meaning of the accessory chromosome. If a large num- 

 ber of observations show variations that parallel well-marked 

 instances of unusual sex characters, then greatly increased prob- 

 ability will attach to the theory I have advanced. 



(/) Concerning the bearing of parthenogenesis upon the prob- 

 lem of sex determination, we know little. In eggs, no struc- 

 ture comparable to the accessory chromosome has yet been ob- 

 served and the presence of any such element is extremely im- 

 probable. But it is known that different sexes come from 

 parthenogenetic eggs, and in the familiar example of the aphides, 

 these are produced in strict response to environmental demands. 



Parthenogenesis, however, is regarded as a degenerate method 

 of sexual reproduction in which polar bodies perform the function 

 of the spermatozoa. Sex might, therefore, be determined by 

 the particular polar body that restored the needed amount of 

 chromatin to the egg, for these, like the spermatozoa, would be 

 of two kinds where a reduction division took place in the process 

 of maturation. These facts would indicate an element of truth 

 in Minot's view regarding the meaning of the polar bodies. In 

 respect to this matter, however, we have only theory to guide us 

 and must wait for more thorough study of the question. 



The suggested hypothesis affords a reasonable basis for a 

 number of theories that have been advanced and supported upon 

 empirical data. Among these are Thury's and Busing's on the 

 time of fertilization ; the ones relating to the nutrition of the 

 parents and embryo ; and possibly others in which age or " com- 

 parative vigor" is assigned as the influential factor. 



In general, I would point out, my theory confirms these by 

 showing that the condition of the ovum determines which sort of 

 spermatozoon shall be allowed entrance into the egg substance. 

 In this we see an extension, to its ultimate limit, of the well- 

 known role of selection on the part of the female organism. 

 The ovum is thus placed in a delicate adjustment with regard to 

 surrounding conditions and reacts in such a way as to best sub- 



