1 86 H. H. NEWMAN AND J. THOS. PATTERSON. 



with reference to one another and to the uterine wall, would be 

 in itself a remarkable coincidence. The chief difficulty, however, 

 in the way of accepting the view that the chorionic vesicle is a 

 multiple structure lies not so much in the interpretation of the 

 relation existing between the foetal membranes, nor in the dis- 

 covery of the mechanics involved in the attainment of such a 

 relation, as in the explanation of the fact that the embryos of any 

 given set are always practically identical and of the same sex. 

 At first sight it might seem that this identity could be explained 

 by the fact that the embryos are apparently under the same 

 environmental influences, inasmuch as they are enclosed within 

 a single chorion. But in reality they are not under exactly the 

 same influences, for each embryo has its own amnion, and is, 

 therefore, surrounded by a fluid apparently as distinct as though 

 each were in a separate chorion. This forces us to the conviction 

 that the four embryos of a set are derived from a single egg. 



If this is the case, one can only conjecture as to the manner 

 in which the conditions seen in the older stages have arisen, and 

 any suggestions which we may have to offer at this time must 

 be taken as tentative. It may be supposed, for example, that 

 the developing egg, after it has reached a stage corresponding to 

 that of an inner-cell mass and trophoblast in other mammals, 

 has, in the case of Dasypns, four inner-cell masses a cell mass 

 for each quadrant ; and furthermore that the cells of any given 

 inner-cell mass, together with the trophoblastic cells of its 'quad- 

 rant, are the lineal descendants of one of the blastomeres of the 

 four-cell stage. This possible interpretation receives a striking 

 confirmation in the fact that the four embryos can be arranged 

 into two pairs, the individuals of which approach almost com- 

 plete identity ; and these identicals are not only adjacent to each 

 other, but are also attached to placental areas that are closely 

 united (see Figs. I and 2 and table). If all four embryos are 

 derived from a single egg, this is exactly what we should expect 

 to find ; for surely the two individuals derived from one of the 

 blastomeres of the two-cell stage ought to be more nearly sim- 

 ilar to each other than to the individuals of the other blastomere. 

 If the above be granted, it is an easy matter to explain the origin 

 of the conditions found in late stages ; and the condition of a 



