No.561] THE NINE-BANDED ARMADILLO 



535 



rate of development frequently occur, as is evidenced by 

 the facts that one pair of fetuses is often strikingly 

 larger than the other. If sex is capable of being altered 

 by nutritive factors one would expect to note some differ- 

 ences of sex within a set in which some fetu>c> have 

 evidently had a much less favorable developmental envi- 

 ronment than others. There are in my collection several 

 sets of quadruplets in which one pair of fetuses is very 

 decidedly larger and more advanced than the other. A 

 condition of this sort is probably to be traced back to a 

 very early period, as early as that shown in Fig. 5, where 

 it is readily seen that one pair is distinctly in advance of 

 the other. Patterson has also stated that it is not un- 

 common to find one of the primary bud primordia divid- 

 ing in advance of the other. If sex is capable of being 

 influenced by metabolic inequalities of any sort, there 

 should be opportunity here for the operation of such 

 influence. Yet there is not a single instance in which 

 there is any diversity of sex within a set of fetuses 

 derived from a single germ cell. 



Cytological studies of the germ cells are in strict accord 

 with current chromosomal hypotheses of sex determina- 

 tion. The female diploid number of chromosomes is 32 

 and the haploid 16; the male diploid 31, producing two 

 kinds of spermatozoa, one with 15 and the other with 16 

 chromosomes. There occurs in the reduction division an 

 odd chromosome like that described for other vertebrates, 

 notably the birds and man as shown by Guyer. The pre- 

 sumption is that this odd chromosome plays the same 

 role in the determination of sex as it is assumed to play 

 in an extensive array of animals. The character of the 

 evidence is the same in all cases. On this basis it may be 

 claimed that in the armadillo an egg fertilized by the 15 

 chromosome type of spermatozoon produces a male and 

 one fertilized by a 16 chromosome type, a female. Envi- 

 ronmental factors are powerless to alter the sex thus 



