384 H. H. Newman and J. T. Patterson. 



tance (fig. 25, en'). This is doubtless only an expression of the 

 same process noted in the study of cross section, in which it was 

 seen that the entoderm was growing in beneath the notochord. 

 The primitive streak has become greatly reduced, due to its 

 transformation into the embryo. The final change to which we 

 would call attention is seen in the great reduction in the length 

 of the allantoic entoderm (al) . It is now not more than one-half 

 of its former length, and is soon destined completely to disappear. 



V. HISTORY OF THE PLACENTA 



Certain isolated stages in the development of the placenta have 

 been described for at least three species of armadillo. 



Kolliker (76), Milne-Edwards (78), and Duges (79-'80), 

 successively described the placental conditions seen in rather 

 advanced vesicles of the South American nine-banded armadillo. 

 Of these accounts that of Milne-Edwards appears to be themost 

 detailed. The embryonic vesicle is described as being a pear- 

 shaped body covered with a chorion, the proximal and distal 

 parts of which were thin and membranous, while the middle part 

 formed a thick, vascular, four-scalloped ring, composed of four 

 fused placentae. 



A stage similar to that just cited was recently described in 

 somewhat greater detail by the present writers, ( ; 09), and illus- 

 trated with two diagrammatic figures. This description of the 

 North American variety of the species seems to agree closely with 

 that of the South American variety as given by the authors just 

 referred to. No doubt we have essentially the same species on 

 both continents. 



The only other reference to the placentation of Tatu novem- 

 cinctum is that of Lane ('09), who described in some detail the 

 afterbirth of a specimen sent to him from central Texas. 



A more comprehensive account of placental conditions is found 

 for Tatu hybridum. Von Ihering states with reference to an 

 advanced stage of placentation, that there is a zonary placenta 

 which has nothing in common with that of the carnivora, but must 

 be considered as a "placenta annularis composita." Each of the 



