The President's Address. By J. Arthur Thomson. 143 



of parents, and so on. These statistics are valuable when the basis 

 is broad enough, but great care is necessary in giving a physio- 

 logical interpretation of statistical results. 



Gytological. — Some generalizations as to the determination of 

 the sex of the offspring have been based on observations of the 

 germ-cells in particular cases. Thus it has been shown that there 

 are two kinds of spermatozoa in some animals, that in some cases 

 one half of the spermatozoa have " an accessory chromosome " 

 absent in the other half, that in some insects ova fertilized by 

 spermatozoa with an accessory chromosome develop into females, 

 and so on. It has also been shown that some animals have two 

 kinds of ova, the larger developing into females. 



Experimental. — Some generalizations as to the determination of 

 the sex of the offspring have been based on experiment, e.g. sub- 

 jecting the eggs, or embryos, or parents, to particular conditions 

 of temperature, nutrition and the like, and observing whether the 

 relative numbers of the sexes in the offspring are in any way 

 different from those obtaining in ordinary conditions ; or by trving 

 particular breeding experiments in reference to what are called 

 sex-limited characters. 



The Chief Theories of Sex-Determination. 



4. Theories on sex-determination may be divided into (a) those 

 which are based on the supposed occurrence of two kinds of germ- 

 cells (male-producing and female-producing), predestined from the 

 beginning and immune to environmental influences ; and (b) those 

 which credit the environment with determining influence, supplying 

 stimuli which give the germ-cell or the developing organism a 

 bias to maleness or to femaleness. 



But I would propose a more detailed classification as clearer, 

 distinguishing five theses : — 



(a) That environmental conditions, operating on the sexually 

 undetermined developing offspring-organism (after fertilization), 

 may at least share in determining the sex ; 



(b) That the sex is quite unpredestined in the germ-cells before 

 fertilization, and that then it is settled by the relative condition of 

 the gametes, or by a balancing of the tendencies they bear, neither 

 gamete being necessarily decisive ; 



(c) That the sex is predestined at a very early stage by the 

 constitution of the germ-cells as such, there being female-producing 

 and male-producing germ-cells, predetermined from the beginning, 

 and arising independently of environmental influence ; 



(d) That maleness and femaleness are Mendelian characters ; 



(e) That environmental and functional influences, operating 

 through the parent (or, in short, the parent's acquired peculiarities), 

 may alter the proportion of effective female-producing and male- 

 producing germ-cells. 



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