EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 15 ^ 



toneal epithelium is continued backwards as a duct (segmental 

 duct), and that the anterior opening subsequently becomes divided 

 up into the various apertures of the head-kidnej. If this account 

 is correct, Petromyzon presents a type intermediate between 

 Amphibia and Elasmobranchii. In certain types, viz. Marsipo- 

 branchii and Teleostei, the segmental duct becomes the duct for 

 the posterior kidney (segmental tubes), but otherwise undergoes 

 no further differentiation. In the majority of types, however, the 

 case is different. In Amphibia,^ as has already been mentioned, 

 a solid rod of cells is split off from its ventral wall, which after- 

 wards becomes hollow, acquires an opening into the body cavity, 

 and forms the Miillerian duct. 



In Elasmobranchii the segmental duct undergoes a more or 

 less similar division. " It becomes longitudinally split into two 

 complete ducts in the female, and one complete duct and parts of 

 a second in the male. The resulting ducts are the (1) Wolffian 

 duct dorsally, which remains continuous with the excretory 

 tubules of the kidney, and ventrally (2) the oviduct or Miillerian 

 duct in the female, and the rudiments of this duct in the male. 

 In the female the formation of these ducts takes place by a nearly 

 solid rod of cells, being gradually split off from the ventral side of 

 all but the foremost part of the original segmental duct, with 

 the short undivided anterior part of which duct it is continuous 

 in front. Into it a very small portion of the lumen of the original 

 segmental duct is perhaps continued. The remainder of the 

 segmental duct (after the loss of its anterior section and the part 

 split off from its ventral side) forms the Wolffian duct. The 

 process of formation of the ducts in the male chiefly differs from 

 that in the female, in the fact of the anterior undivided part of 

 the segmental duct, which forms the front end of the Miillerian 

 duct, being shorter, and in the column of cells with which it is 

 continuous being from the first incomplete." 



It will be seen from the above that the Miillerian duct consists 

 of two distinct parts — an anterior part with the abdominal open- 

 ing, and a posterior part split off from the segmental duct. This 

 double constitution of the Miillerian duct is of great importance 

 for a proper understanding of what takes place in the Bird. 



The Miillerian duct appears, therefore, to develop in nearly the 

 same manner in the Amphibian and Elasmobranch type as a 

 solid or nearly solid rod split off from the ventral wall of the 

 segmental duct. But there is one important difference concern- 

 ing the abdominal opening of the duct. In Amphibia this is a 

 new formation, but in Elasmobranchii it is the original open- 

 ing of the segmental duct. Although we admit that in a large 

 number of points, including the presence of a head-kidney, the 



^ Fiirbringer, loc. cit. 



