238 Vertebrate Embryology 



actually formed. We shall use the term " Wolff- 

 ian ridge " for the entire mass of mesoblast that 

 projects into the upper part of the body-cavity, 

 and that forms the externally visible ridge 

 that has already been spoken of as the Wolff- 

 ian ridge (page 223). 



The whole Wolffian ridge is for a time 

 covered evenly with a single layer of columnar 

 epithelium, which may even extend, for a short 

 distance, over the adjacent parts of the so- 

 matopleure and splanchnopleure (Fig. 70). 

 The central part of the Wolffian ridge is 

 occupied by the Wolffian body and duct, and 

 it is, in fact, the increase in size of the former 

 that is the chief cause of the increase in prom- 

 inence of the Wolffian ridge (Fig. 74). The 

 epithelial cells covering this middle portion of 

 the ridge rapidly lose their columnar character 

 and become flattened. The cells of the outer 

 part of the ridge, next to the somatopleure, 

 retain for a longer time their columnar char- 

 acter, and it is here that the involution to 

 form the Miillerian duct takes place. 



At the inner angle of the Wolffian ridge, 

 next to the splanchnopleure, the columnar cells, 

 instead of diminishing in distinctness, become 

 more distinct, and even become several layers 



