ZOOLOGICAL SCIENCE 205 



development proceeded, the individual became a male, if it 

 happened to receive a scanty diet; a female, if it chanced to be 

 well fed. Experiments in overfeeding and in underfeeding of 

 the young of vertebrates like the frog and of invertebrates like 

 the moth gave what many regarded as conclusive evidence. 



But the experiments in feeding, which were supposed to 

 have thus determined the sex, have been repeated in recent 

 years, with results that do not confirm the earlier conclu- 

 sions. 17 Moreover, it has been found that the sex of many 

 animals is seemingly determined as early as the stage when 

 the individual originates by the union of egg and spermato- 

 zoon. The individual becomes a male or a female at the very 

 beginning. Nothing that happens in the subsequent devel- 

 opment changes the sex as thus early established. 



The facts upon which the new theory rests may be illus- 

 trated by Fig. 27. As we have seen (Fig. 13), the nuclei 

 of cells exhibit at the time of division certain bodies, the 

 chromosomes, which occur in pairs (Fig. 16) and in numbers 

 that are constant for a given species. In the present in- 

 stance four pairs is the number chosen for the purposes of 

 the diagram (Fig. 27). It has been observed, in numerous 

 cases among insects and in a smaller list of other forms, that 

 the number of chromosomes is not the same for the two 

 sexes, since males lack one member of one of the pairs. 18 

 The chromosomes of this particular pair are termed "X" 

 chromosomes, or better sex chromosomes, because their 

 distribution at the time of fertilization appears to deter- 

 mine the sex of the individual. Thus, the Pi adults of 



17 In the language of one investigator, these later experiments "seem to show 

 that sex is not determined by the quantity or the quality of the food that the 

 larva? receive." This conclusion was based upon experiments with tadpoles 

 of the toad. It agrees with that reached by other investigators who have 

 reinvestigated the influence of food upon sex in frogs, moths, and other forms. 

 King, H. D., "Food as a Factor in the Determination of Sex in Amphibians," 

 Bid. Bulletin, Vol. XIII, No. 1, June, 1907. 



18 In a few cases the condition is reversed and it is the female that lacks one 

 chromosome several moths and butterflies and several birds. 



