THE URTNOGENITAL ORGANS OF VERTEBRA IKS. l6l 



there are no essential variations except in the later periods 

 of development throughout the division. These later varia- 

 tions, concerning for the most part the external apertures of 

 the various ducts, are so well known and have been so fully 

 described as to require no notice here. The development of 

 these parts in the bird will therefore serve as the most conve- 

 nient basis for comparison. 



In the bird the development of these parts begins by the 

 appearance of a column of cells on the upper surface of the 

 intermediate cell-mass (Fig. 8, W.cT). As in Selachians, the in- 

 termediate cell-mass is a group of cells between the outer edge 

 of the protovertebrae and the upper end of the body cavity. 

 The column of cells thus formed is the commencement of the 

 duct of the Wolffian body. Its development is strikingly similar 

 to that of the segmental duct of the kidney in Selachians. I 

 shall attempt when I have given an account of the development 

 of the Miillerian duct to speak of the relations between the 

 Selachian duct and that of the bird. 



Romiti (ArcJiiv f. Micr. Anat. Vol. X.) has recently stated 

 that the Wolffian duct developes as an involution from the 

 body cavity. The fact that the specimens drawn by Romiti 

 to support this view are too old to determine such a point, and 

 the inspection of a number of specimens made by my friend 

 Mr Adam Sedgwick of Trinity College, who, at my request, 

 has been examining the urinogenital organs of the fowl, have 

 led me to the conclusion that Romiti is in error in differing 

 from his predecessors as to the development of the Wolffian 

 duct. The solid string of cells to form the Wolffian duct lies 

 at first close to the epiblast, but, by the alteration in shape which 

 the protovertebrae undergo and the general growth of cells 

 around it, becomes gradually carried downwards till it lies close 

 to the germinal epithelium which lines the body cavity. While 

 undergoing this change of position it also acquires a lumen, 

 but ends blindly both in front and behind. Towards the end 

 of the fourth day the Wolffian duct opens into a horn of 

 the cloaca. The cells adjoining its inner border commence, 

 as it passes down on the third day, to undergo histological 

 changes, which, by the fourth day, result in the formation of a 

 B. II 



