THE EMBRYOLOGY OF THE OPOSSUM 107 



and which also has a well-developed primary lumbar flexure, 

 though here the picture is somewhat complicated by the fact 

 that the anterior part of the body, being unable to dip into 

 the dense yolk, is forced to twist to one side. The rabbit and 

 the lizard, which do not show the flexure, also do not have 

 any mesonephric tubules developing at this time. In these 

 cases the tubules form later on when there are other forces at 

 work which compel the mesonephros to expand by projecting 

 into the coelom instead of by flexing the back. The 'other 

 forces' referred to will be discussed in connection with 

 stage 30. 



The mesonephric tubules. In the description of the nephro- 

 genic ridge during late stage 24 the earliest anlagen of the 

 mesonephric tubules were mentioned (p. 67 and fig. 22). The 

 anlagen at that time were differentiating within a continuous 

 longitudinal ridge. They consisted simply of a series of small, 

 separate lumina along the center of the thickest part of the 

 ridge. The cells around the lumina were radially disposed, 

 but between adjacent lumina the cells showed no recognizable 

 organization. 



In stage 28 two important developments take place. First, 

 the ridge pinches off from the coelomic epithelium altogether, 

 thus becoming a nephrogenic cord. And second, the meso- 

 nephric tubule anlagen begin to separate from each other, in- 

 crease rapidly in length, and become bent into double curves 

 to accommodate themselves to the inadequate, available space. 

 Though the adjacent tubule anlagen separate from one 

 another (probably by the organization of the intermediate 

 cells about the separate lumina), they do not separate from 

 the most dorsal part of the original ridge. This once dorsal 

 edge has now become lateral as a result of the rounding up of 

 the body. Though it itself remains unsegmented and for the 

 most part at stage 28 still uncanalized, it never loses con- 

 tinuity with the walls of the tubule anlagen which differentiate 

 out of the same longitudinal anlage. It is, of course, the 

 wolffian duct, which is thus continuous with all of the pro- 

 nephric and mesonephric tubules from the very beginning. 



