THE EMBRYOLOGY OF THE OPOSSUM 137 



crypts of the uterine mucosa (fig. 17, also Hartman, '23, 

 fig. 31). These folds and crypts become exceedingly compli- 

 cated as development continues, but no fusion between foetal 

 and maternal tissues ever occurs. The use of the extensive 

 folding is only to increase the surface of contact. At any time 

 during development the vesicles may be separated from the 

 mucosa without a tear and without loss of either foetal or 

 maternal blood. 



The non-vascular portions of the vesicle enlarge by ex- 

 panding medially into the lumen of the uterus and laterally 

 until tliev meet each other. The surfaces of lateral contact 



Fig. 44 Photographs of external views of stage 31. A, lateral view of 16164 

 photographed alive in Ringer's solution. B, lateral view of 16162 with all extra- 

 embryonic membranes except the allantois removed. C, ventral view of same 

 specimen. 



of adjoining vesicles then fuse together so that each vesicle 

 assumes the shape roughly indicated in figure 45, G. The 

 vesicles thus develop non-vascular, membranous attachments 

 to each other, but never any sort of attachment to the maternal 

 tissues. All of the changes just recounted were accurately 

 described by Selenka. 



Changes in the venous system. The omphalomesenteric 

 veins have enlarged enormously. The left side of the anterior 

 loop, which was plexiform in stage 30, is altogether gone in 

 stage 31, so that there is only a single large vein at the level 

 of the stomach, which passes dorsad around it and up to the 



