Development of the Nine-Banded Armadillo. 409 



There is on the other hand excellent evidence that the male cell 

 may act as a sex determiner. Studies of the spermatogenesis 

 of our species show that the spermatogonial number of chromo- 

 somes is in all probability 31, one less than the oogonial. There 

 is moreover in the reduction division a very definite and obvious 

 odd chromosome, which precedes the other chromosomes to the 

 pole of the spindle and serves to institute a dimorphism of the 

 spermatids. That the odd chromosome is concerned with the 

 determination of sex is as probable for the armadillo as for the 

 insects and other forms in which it has been described. Both 

 rest on the same observations. Since it is our intention to make 

 a detailed study of the cytology of the germ cells in this species, 

 it must suffice for the present to have indicated the sort of 

 external, evidence of polyembryony and of sex determination we 

 have at our command. 



The discovery of so clear a case of an accessory chromosome 

 in a mammal is in itself worthy of mention, since it brings us a 

 few steps nearer to the discovery of the physiology of sex deter- 

 mination in man. In addition to the intrinsic value of this dis- 

 covery, however, we are afforded another strong proof of specific 

 polyembryony, in that it is highly improbable, on the basis of 

 the origin of the embryos of a vesicle from several fertilized eggs 

 that each of these eggs would be fertilized by the same kind of 

 spermatozoon. Such a possibility could be realized only through 

 the instrumentality of selective fertilization, the occurrence of 

 of which has never been successfully demonstrated. 



XII. SUMMARY OF EVIDENCE FOR SPECIFIC POLYEMBRYONY 



1. The uterus is simple, resembling that of the primates, which 

 give birth typically to one offspring at a time. 



2. There is never more than one large corpus luteum in the 

 ovaries of a pregnant female. 



3. In over 90 per cent of vesicles the number of normal embryos 

 is four, a number that suggests their origin from the blastomeres 

 of the four-cell stage. It is also unlikely that this number of ova 

 would so often be given off at the same time. 



