12 BALFOUR AND SEDGWICK. 



followed the last phases of its growth and its final opening 

 into the cloaca. 



In some of our embryos we have noticed certain rather pecu- 

 liar structures, an example of which is represented at y in fig. k, 

 taken from an embryo of 123 hours, in which all traces of the 

 head-kidney had disappeared. It consists of a cord of cells, 

 connecting the Wolffian duct and the hind end of the abdominal 

 opening of the Miillerian duct. At the least one similar cord 

 was met with in the same embryo, situated just behind the ab- 

 dominal opening of the Miillerian duct. We have found similar 

 structures in other embryos of about the same age, though never 

 so well marked as in the embryo from which fig. k is taken. 

 We have quite failed to make out the meaning, if any, of them. 



Our interpretation of the appearances we have described in 

 connection with the growth of the Miillerian duct can be stated 

 in a very few words. Our second stage, where the solid point 

 of the Miillerian duct terminates by fusing with the walls of 

 the Wolffian duct, we interpret as meaning that the Miillerian 

 is growing backwards as a solid rod of cells, split off from the 

 outer wall of the Wolffian duct ; in the same manner, in fact, as 

 in Amphibia and Elasmobranchii. The condition of the terminal 

 part of the Miillerian duct during our third stage cannot, we 

 think, be interpreted in the same way, but the pecuHarities of the 

 cells of both Miillerian and Wolffian ducts, and the indistinctness 

 of the outlines between them, appear to indicate that the 

 Miillerian duct grows by cells passing from the Wolffian duct 

 to it. In fact, although in a certain sense the growth of the 

 two ducts is independent, yet the actual cells which assist in 

 the growth of the Miillerian duct are, we believe, derived from 

 the walls of the Wolffian duct. 



III. 



General Considerations. 



The excretory system of a typical Vertebrate consists of the 

 following parts : 



1. A head-kidney with the characters already described. 



2. A duct for the head-kidney — the segmental duct. 



3. A posterior kidney — (Wolffian body, permanent kidney, &c. 

 The nature and relation of these parts we leave out of considera- 

 tion, as they have no bearing upon our present investigations.) 

 The primitive duct for the Wolffian body is the segmental duct. 



4. The segmental duct may become split into {a) a dorsal or 

 inner duct, which serves as ureter (in the widest sense of the 

 word) ; and {h) a ventral or outer duct^ which has an opening 



