ZOOLOGICAL SCIENCE 209 



If sex is determined by the kind of spermatozoon which 

 happens to fertilize a particular egg (Fig. 27), the difficulties 

 in the way of sex control seem insurmountable. There ap- 

 pears no immediate prospect of controlling sex production 

 in man and the domesticated animals. But something has 

 been gained if it becomes clear that we cannot alter the mole- 

 ness or femaleness, which is established at the time of fertili- 

 zation, by changing the food or environment of subsequent 

 stages. The point of attack is known, even though control 

 seems well-nigh impossible in the event that sex is irrevoc- 

 ably determined at the time of fertilization. Investigation 

 is pressing so closely upon the solution of the whole question 

 that the factors controlling the union of the germ-cells in 

 fertilization, and hence the sex of the individual, may be 

 discovered sooner than one expects. Control of sex in man 

 and the domesticated animals is, therefore, a remote, though 

 not an unthinkable possibility. 21 



The changes from the origin of an individual at the time 

 of fertilization to its disintegration in death are a never- 

 ending wonder to the advanced investigator as well as to the 

 novice. So manifold has been the work of determining the 

 structural changes, by which the egg becomes the adult, 

 that embryology has but recently begun the experimental 

 analysis of causation in development which is illustrated by 

 the work above described. In this more intensive study, 

 experiment and observation must, of course, be inextric- 

 ably interwoven. Never the less the major problem hence- 

 forth must be discovery of underlying causation, rather 

 than description of structural sequence. 



21 Although sex-determination by means of the sex-chromosome appears to 

 be well established in certain animal types, there may be exceptions to the 

 established scheme. Also, there may be other factors which have a deter- 

 minative action. The studies of Riddle upon the pigeon and those of Whitney 

 and of A. F. Shull upon rotifers, together with the work of F. R. Lillie upon 

 sex-hormones in cattle, illustrate the complexity of the problem and should 

 make us hesitate to accept the chromosome-theory as a universal explanation 

 of the determination of sex. 



