IN THE EMBRYO CHICK. 635 



however, the case is different. In Amphibia 1 , as has already 

 been mentioned, a solid rod of cells is split off from its ventral 

 wall, which afterwards becomes hollow, acquires an opening into 

 the body-cavity, and forms the Mullerian 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 (i) the Wolf- 

 fian duct dorsally, which remains continuous with the excretory 

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

 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 con- 

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

 the original segmental duct is perhaps continued. The re- 

 mainder 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 Mullerian duct, being shorter, and in the column 

 of cells with which it is continuous being from the first incom- 

 plete." 



It will be seen from the above that the Mullerian duct con- 

 sists of two distinct parts an anterior part with the abdominal 

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

 This double constitution of the Mullerian duct is of great im- 

 portance for a proper understanding of what takes place in the 

 Bird. 



The Mullerian 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 opening 

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



1 Fiirbringer, loc. fit. 



412 



