530 LIFE : OUTLINES OF GENERAL BIOLOGY 



tozoa are female-producers, and every one knows that all the 

 fertilised ova produce females. An interesting correlated fact is that 

 in Phylloxera and Aphides the males have in their bodies one chro- 

 mosome fewer than the females have. "The male-producing egg", 

 Wilson notes, "must therefore eliminate one chromosome, and this, 

 we cannot doubt, is the X-element." 



Investigations by Guyer and others point to the conclusion that 

 in various Vertebrates, e.g. fowl, guinea-pig, rat, and man, the 

 male has one chromosome less than the female. Thus man has 47 

 and woman 48. 



The theory that tlie presence of one X-element in a fertiUsed 

 ovum means male offspring, and that the presence of two means 

 female off.spring is morphological and leaves our physiological sense 

 unsatisfied. Is the difference significant in itself, or as an index of 

 differences in metabolism? If the eggs with more chromatin than 

 their neighbours develop into females and if chromatin be an index 

 of a relatively prejx)nderant anabolic capacity, cannot the theory 

 be brought into line with the thesis of The Evolution of Sex, 

 that the female is the outcome and expression of relatively pre- 

 ponderant anaboUsm, and the male of relatively preponderant 

 katabolism? 



Baltzer observed that about half of the eggs of the sea-urchin 

 are distinguished from the others by having one of the eighteen 

 chromosomes represented by a short "hook-chromosome" instead 

 of a normal "rod-chromosome", and there is indirect evidence that 

 those ova with the short "hook-chromosome" develop into males. 

 In discussing this result and comparing it with the state of affairs 

 in the various insects already referred to, Boveri points out that in 

 both sets of cases the fertilised ovum from which a female develops 

 has more chromatiyi than that from which a male develops, and that 

 the amount of chromatin has a regulative influence on the amount 

 of cytoplasm. He suggests that in some cases nurtural influences 

 operate variably or unequally on sexually indifferent germ-cells, 

 giving them a bias to the one sex or the other, and that in other 

 cases the decision is due to an internal factor such as the presence 

 of stronger "a.ssimilation-chromosomes" in some of the ova. This 

 is in line with the thesis in our Evolution of Sex (1889). 



On the other hand, it may be that the additional chromatin 

 material is of qualitative importance. Thus Prof. E. B. Wilson 

 suggests, quite provisionally, that the X-element contains factors 

 (enzymes or hormones^) that are necessary for the production of 

 both the male and the female characters; that these are so adjusted 

 that in the presence of a single X-element the maleness character 

 dominates, or is set free; and that the association of two such 

 elements lead^' to a reaction which activates the femaleness 

 character. 



