POLYEMBRYONIC DEVELOPMENT IN TATUSIA 593 



The cavity has been previously termed the 'common amniotic 

 cavity' (Fernandez '09, and Newman and Patterson '10), because 

 it is common to the amniotic connections of the four embryos 

 which later take their origin from the ectodermal vesicle. 



It is evident from this that in the mode of amnion formation the 

 armadillo is to be classed with that group of mammals in which 

 the amniotic cavity is from the first an enclosed space, and never 

 has a free communication with the space outside the trophoblast. 

 The cavity is always intra-trophoblastic (Hubrecht '08), and 

 consequently no folds ever arise to deliminate it, for this is not 

 necessary. The armadillo can therefore be added to Hubrecht's 

 ('08) compiled list of mammals, in which the amniotic cavity arises 

 within the ectoderm and remains a closed vesicle. His list is as 

 follows": "Cavia and other rodents, Pteropus, Galeopithecus, 

 Erinaceus, Gymnura, monkeys and man. 5 



ORIGIN OF THE EXTRAEMBRYONIC MESODERM OR 

 MESOTHELIUM 



The origin of the mesoderm in mammals is a problem which has 

 given rise to much difference of opinion among embryologists, but 

 it is not necessary here to enter into this controversy. The first 

 appearance of the mesoderm in the armadillo blastocyst is seen in 

 connection with the cavity which lies between the ectodermal 

 vesicle and the placenta, or what has been termed above, the extra- 

 embryonic cavity. 



The entoderm, at the angle where it parts company with the 

 ectodermal sphere to join the trophoblast (fig. 51), gives rise to a 

 few cells which frequently become scattered throughout the extra- 

 embryonic cavity. There is good reason for believing that most 

 of these cells undergo disintegration pari passu with the develop- 

 ment of the ectodermal vesicle. At any rate there comes a tune 

 when the exocoelomic space is essentially free from such ento- 

 dermal cells (fig. 54) . In some of the sections of this series a few 

 of these cells are still found within the cavity. Thus in figure 55 

 there are four of them, two lying some distance below the ectoderm 

 and slightly to the left of the center, and two situated against the 



5 Loc. cit., p. 71. 



