258 CONCLUSION [CH. 



hypothesis that sex is determined by specific chromosomes ; 

 the first suggestion on the subject came from McCLUNG who 

 was led to it from the study of the "accessory chromosome" 

 in various insects. In general it is only after cytologists 

 have discovered a possible basis for the explanation of some 

 general function that a definite search can be made, with 

 any prospect of success, for cytological facts that shall 

 explain some specific phenomenon, for unless the investiga- 

 tor has some idea of what to expect, he is hardly likely 

 to find his object. When, however, other facts are known 

 which make some definite condition seem possible or prob- 

 able in a specific case, cytological investigation may lead 

 to the explanation sought for. Recent examples of this sort 

 are the discovery by BRIDGES of non-disjunction of the sex- 

 chromosomes as an explanation of the exceptions to sex- 

 limited inheritance in Drosopbila, and the finding of an 

 unpaired or unequally paired chromosome in the females 

 of certain Lepidoptera. In each of these cases the facts 

 of heredity seemed to demand a definite cytological con- 

 dition, and when examination was made the expected 

 condition was found. 



The study of heredity and sex-determination has had so 

 large a share of recent biological research that it is not 

 surprising that the attention of cytologists should have been 

 especially attracted to this aspect of the subject, and this 

 tendency has been accentuated by the comparative ease 

 with which chromosomes are observed and the beauty of 

 the mitotic process. There are, however, other directions 

 in which research is no less needed. One of the most im- 

 portant is that of cell-differentiation. As was pointed out 

 in the last chapter, the problem of differentiation is one of 

 the fundamental problems of life; on the answer to the 

 question of how differentiation is effected depends, to a great 



