S6 THE BIOLOGY OF TWINS 



these two points we are able to show that the arrange- 

 ment of embryos is not precisely in accord with the 

 uterine bilaterality. Embryos II and IV are not 

 exactly lateral, nor are embryos I and III strictly ventral 

 and dorsal respectively. About a third of the egg at 

 the distal (lower) end is very thin-walled and trans- 

 parent, owing to the fact that it is composed — except 

 where the amnia are fused with it— of but two layers of 

 cells, endoderm externally and mespthelium internally. 

 One can look into this part of the egg as through a 

 window. 



Stage X. Five- and seven-somite embryos in an egg 

 with early true placenta (Fig. i8). — Several points of 

 interest are shown in this figure, which is redrawn from 

 one previously published in which an inadvertent error 

 was made.' The placenta now consists of a broad band 

 of villi that had penetrated the mucosa to such an 

 extent that it required some effort to pull the egg free. 

 The villi are rather flat and tend to overlap like shingles. 

 Each embryo is coimected with the placenta by means 

 of a double primitive umbilicus, which as yet is not 

 traversed by blood vessels. The allantois is, as before, 

 vestigial. The two primary embryos (II and IV), those 

 nearer the lateral edges of the blastocyst, are more 

 advanced than the two secondary individuals (I and 

 III) ; this is due to the fact that the former are some- 

 what older than the latter. The bilateral arrangement 

 of the egg is still preserved, as in stage IX, by the 

 lateral horns or bumps that represent the points of 

 protrusion of the vesicle wall into the Fallopian tubes. 



' In drawing in the details of the embryos the two upper embryos 

 were formerly reversed in position. TMs redrawing corrects the error. 



