THE EMBRYOLOGY OF THE OPOSSUM 65 



Tafel XXI, and they were very likely also present in his 

 slightly younger specimen shown in Tafel XX. In sections, 

 of course, there is not the slightest difficulty in identifying 

 them (fig. 20, A). 



The acoustic placode (fig. 21) is situated on the dorso- 

 caudal surface of the hyoid swelling just anterior to the 

 second branchial groove (v.i.). This section (17153, 13,4,12) 

 passes through the anterior portion of the placode, which is 

 flat, but figure 20, E, shows that posteriorly the placode 

 wrinkles under the edge of the medullary plate. 



For the discussion of the sensory ganglia see the section 

 on the medullary plate and the neural crest. 



The nephrogenic ridge. The pronephros in the opossum is 

 no better developed than in other mammals, and no less 



mp. 



n.c. 



Fig. 21 Section through otic placodes and medullary plate at stage 24 

 (17153, 13, 4, 12). 



well developed than in the lower amniotes. At the height of 

 its development it is composed of only a few very small, solid 

 tubule anlagen attached at one end to the coelomic epithelium 

 and at the other end to a solid duct. As the tubules are solid 

 no true nephrostomes appear, though in many cases a small 

 indentation in the coelomic epithelium at the point of origin 

 of the tubule may represent the vestige of one. The develop- 

 ment of the pronephros, however, has acquired a renewed 

 interest in the light of the theory recently proposed by Bur- 

 lend ('31), which will be discussed below; and the details of 

 this development in the opossum turn out to be significant 

 in connection with this theory. 



Of the six specimens representing stage 24 in The Wistar 

 Institute collection, all but one show definite signs of nephro- 

 genesis. The one exceptional embryo (17166) is abnormal in 



