190 CHAELES R. STOCKARD 



d. An explanation of polyemhryony in the armadillo 



On examining the uterus in two pregnant specimens of a South 

 American armadillo, von Jhering in 1885 discovered that each 

 contained eight fetuses enclosed within a single chorion. He cor- 

 rectly concluded that all of the fetuses in each mother had been 

 derived from a single egg by some process of division into separate 

 embryonic rudiments. After this valuable discovery and inter- 

 pretation, the study of the armadillo's development lapsed and 

 nothing of importance was added for almost twenty-five years. 

 Two series of investigations were then begun simultaneously, one 

 on the South American species by Fernandez ('09) and the other 

 on the Texas armadillo by Newman and Patterson ('09). The 

 growth and expansion of these twin studies has brought our 

 understanding of the phenomenon of polyembryonj^ in the arma- 

 dillo to a considerable state of maturity. 



These authors readily agreed that in most species of armadillo 

 the individual members of a litter, usually four in the Texas 

 species and eight in the common South American form, are all 

 derived from a single egg. It required considerable effort, how- 

 ever, to obtain the material that would furnish the morphological 

 stages of the proccess by which this polyembryonic development 

 was accomplished. We are finally indebted to Patterson ('13) for 

 the very thorough and satisfactory manner in which he has col- 

 lected and studied the early embryonic conditions; and particu- 

 larly for having shown the first stages of the budding process 

 through which the single blastocyst gives rise to four distinct 

 embryonic areas, each exhibiting a typical primitive streak region. 



In connection with the idea constantly advanced in the present 

 study that twins and double vertebrate embryos arise from ac- 

 cessory growths or invagination points around the blastoderm, 

 it now becomes important to ascertain exactly what degree of 

 development has been attained by the armadillo blastocyst at the 

 time the budding process begins. And since, according to our 

 interpretation, these buds should arise at the time of gastrula- 

 tion or blastopore formation, it becomes necessary to consider 

 very briefly the germ-layers and gastrulation in mammals. The 



