IN THE EMBRYO CHICK. 63! 



its outer wall on the level of the Mullerian duct comes into con- 

 tact with the germinal epithelium (Plate 28, series I, No. 5). 



We have traced the growing point of the Miillerian duct with 

 the above features till not far from the cloaca, but we have not 

 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 

 abdominal opening of the Miillerian duct. We have found simi- 

 lar 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 peculiarities of the 

 cells of both Mullerian and Wolffian ducts, and the indistinctness 

 of the outlines between them, appear to indicate that the Muller- 

 ian 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 Mullerian duct are, we believe, derived from the walls of the 

 Wolffian duct. 



