Development of the Nine-Banded Armadillo. 407 



following observations : that at a very early period each embryo 

 is surrounded by its own amnion; that a little later each draws 

 maternal nutriment from a separate area of the uterine wall; and 

 that there is no admixture of foetal blood. We are therefore 

 driven to the conclusion that sex is determined before there 

 occurs any splitting of the single germ into separate embryonic 

 primordia. 



Opinions differ as to the exact period at which this splitting 

 takes place. Fernandez maintains, on the basis of his studies of 

 the early blastocyst of the mulita, that there is no trace of poly- 

 embryony until after the two primary germ layers have been 

 laid down. What he probably means is that previous to this time 

 there is no visible demarcation of the germ layers into isolated 

 blastodermic areas. That the real separation of embryonic rudi- 

 ments occurs at a much earlier period, even during the early cleav- 

 age stages (in our species at the four-cell stage), seems probable 

 in view of the discovery of pairing among the embryos, a phenome- 

 non for which no other explanation offers itself; and by the obser- 

 vations of Marchal, ('04), and Silvestri ('06), on the parasitic 

 hymenoptera, where each embryo in a set takes its rise from a 

 single cell of a rather advanced cleavage stage. 



It seems highly probable then that the tissues involved in each 

 of the four quadrants of an embryonic vesicle, whether or not 

 they may show a demarcation, do really arise as the lineal descend- 

 ants of one of the first four blastomeres. In this sense the four 

 embryos are delimited at the four cell stage. It is hardly to be ex- 

 pected that any demarcation would be visible before the beginning 

 of the period when the separate embryonic shields are differen- 

 tiated. 



The question as to the exact period of separation of the several 

 embryonic rudiments is one that cannot at present be definitely 

 settled. Even if one should be fortunate enough to obtain the 

 early cleavage stages it is improbable that he would be able to 

 observe any essential departure from the usual plan of mammalian 

 cleavage, for a blastomere of the four cell stage would have the 

 same appearance whether it were destined to produce a whole 

 or only a quarter of an embryo. 



