AMNIOTA. 



mental duct. It apparently develops in all the Amniota nearly 

 on the Elasmobranch type, as a solid rod, primarily derived 

 from the somatic mesoblast of the intermediate cell mass (fig. 

 401 W.dy. 



The first trace of it is visible in an embryo Chick with eight 

 somites, as a ridge projecting from the intermediate cell mass to- 

 wards the epiblast in the region of the seventh somite. In the 

 course of further development it continues to constitute such a 

 ridge as far as the eleventh somite (Sedgwick), but from this 

 point it grows backwards in the space between the epiblast and 

 mesoblast. In an embryo with fourteen somites a small lumen 

 has appeared in its middle part and in front it is connected with 

 rudimentary Wolffian tubules, which develop in continuity with 

 it (Sedgwick). In the succeeding stages the lumen of the duct 

 gradually extends backwards and forwards, and the duct itself 

 also passes inwards relatively to the epiblast (fig. 402). Its hind- 

 end elongates till it comes into connection with, and opens into, 

 the cloacal section of the hind-gut 2 . 



It might have been anticipated that, as in the lower types, 

 the anterior end of the segmental duct would either open into 

 the body cavity, or come into connection with a pronephros. 

 Neither of these occurrences takes place, though in some types 

 (the Fowl) a structure, which is probably the rudiment of a 

 pronephros, is developed ; it does not however appear till a later 

 stage, and is then unconnected with the segmental duct. The 

 next part of the system to appear is the mesonephros or 

 Wolffian body. 



This is formed in all Amniota as a series of segmental tubes, 

 which in Lacertilia (Braun) correspond with the myotomes, but 

 in Birds and Mammalia are more numerous. 



In Reptilia (Braun, No. 542), the mesonephric tubes develop as seg- 

 mentally-arranged masses on the inner side of the Wolffian duct, and 

 appear to be at first united with the peritoneal epithelium. Each mass soon 

 becomes an oval vesicle, probably opening for a very short period into the 



1 Dansky and Kostenitsch (No. 543) describe the Wolffian duct in the Chick as 

 developing from a groove opening to the peritoneal cavity, which subsequently 

 becomes constricted into a duct. I have never met with specimens such as those 

 figured by these authors. 



2 The foremost extremity of the segmental duct presents, according to Gasser, 

 curious irregularities and an anterior completely isolated portion is often present. 



