216 THE FOURTH DAY. [CHAP. 



Fig. 70. 



Sections shewing two of the Peritoneal Invaginations 



WHICH GIVE rise TO THE ANTERIOR PaRT OP THE MuL- 

 LEHIAN BUCT (PRONEPHROS). 



A is the 11th .section of the series. 



B „ 15th 



C „ 18th „ „ 



gr 2 second groove, gr 3 third groove. )■ 2 second ridge, wd. 

 Wolf&an duct. 



As long as the openings persist, the Miillerian duct 

 consists merely of a very small rudiment, continuous 

 with the hindermost of them, and its solid extremity 

 appears to unite with the walls of the Wolffian duct. 



After the closure of the two hinder openings the 

 Miillerian duct commences to grow rapidly backwards, 

 and for the first part of its subsequent course it 

 appears to be split off as a solid rod from the outer or 

 ventral wall of the Wolffian duct (Fig. 72). Into this 

 rod the lumen, present in its front part, subsequently 

 extends. Its mode of development in front is thus pre- 

 cisely similar to that of the Miillerian duct in Elasmo- 

 branchii and Amphibia. 



This mode of development only occurs however in 

 the anterior part of the duct. In the posterior part of 



