THE EMBRYOLOGY OF THE OPOSSUM 



67 



somite. This includes all of the pronephric region and the 

 beginning of the mesonephric. 



The pronephric anlage extends from a point opposite the 

 middle of the seventh somite to a point opposite the middle 

 of the tenth. There are no separate tubules. The thick, 

 rounded edge of the ridge, which represents the pronephric 

 duct, is continuous antero-posteriorly, and is connected with 

 the coelomic lining laterally by a thinner edge from which the 

 tubules will be differentiated. What breaks there are in the 

 tubular portion at this time bear no relation to the somites, 

 but occur at both somitic and inter-somitic levels. The well- 

 formed break near the beginning of the tenth somite admits 

 the passage of a small artery from the dorsal aorta. 



7* Jomit-e , , 6* Somite 96<5omifo . . JO*<5om'ite. , , I/* Somite. 



Fig. 22 Reconstruction of nephrogenic ridge in 17139. The somite levels 

 are indicated above the reconstruction. The dotted ellipses near the right end 

 of the figure indicate the extent of the lumina of the anlagen of the first four 

 mesonephric tubules. 



Near the beginning of the eleventh somite the thick por- 

 tion of the ridge enlarges very markedly, the dorso-ventral 

 diameter (that perpendicular to the plane of the paper) being 

 particularly increased. This marks the beginning of the 

 mesonephros. In section the more dorsal portion can be 

 recognized as the anlage of the wolffian duct, which does not 

 yet possess a lumen. The more ventral portion already shows 

 some organization into tubules, the lumina of the first four 

 of which are indicated in the reconstruction by dotted lines. 

 But the tubule anlagen have not become detached from each 

 other, or from the somatopleure, or from the wolffian duct. 

 In fact, in the opossum the mesonephric tubules are not at 

 any time separate from the wolffian duct. They are formed 

 from the same anlage. Only the lumina of the duct and the 

 tubules arise independently and later coalesce. The walls of 



