6o 



THE BIOLOGY OF TWINS 



of the blastocyst cavity. The shrunken amniotic 

 connecting canals and the common amnion are quite 

 obvious at this time and persist in stages still more 

 advanced than this. The large area at the bottom of 

 the vesicle, extending from the placenta to the some- 

 ^ what pointed vege]- 



tative pole, retains its 

 transparency. 



It may be noted 

 that from this stage up 

 to a stage shortly be- 

 fore birth no changes 

 occur that are espe- 

 cially significant for a 

 study of twinning. 

 The whole vesicle 

 grows enormously, 

 but the embryo and 

 the amnia increase in 

 volume more rapidly 

 than does the vesicle 

 wall. The result is 

 that the amnia fill the 

 cavity of the blastocyst 

 as is shown in the next 

 figure. 



Stage XII. A full-term quadruplet egg (Fig. 20). — 

 The figure of the vesicle is semi-diagrammatic in detail,^ 



^ Strahle, in a paper entitled "tJber den Bau der Placenta von 

 Dasyurus novemcinctus," gives an incorrect account of the arrangement 

 of the fetuses in the uterus, in that he shows the amniotic partitions 

 coinciding exactly with the dorsoventral and the bilateral axes of the 

 uterus. 



Fig. 20. — Semi-diagrammatic sec- 

 tional view of full-term armadillo egg, 

 seen from the lower pole (cf. Fig. ig). 

 The four fetuses occupy four quad- 

 rants of the egg, with partitions be- 

 tween them composed of the fused 

 amnia of adjacent individuals. The 

 placental areas are two thick bilater- 

 ally arranged double disks, each with 

 the umbilical cords of two fetuses 

 attached. (See stage XII.) 



