IN THE EMBRYO CHICK. 629 



period prior to the disappearance of the head-kidney. At this 

 stage the structure we have already spoken of as the rudiment 

 of the Miillerian duct consists of a solid rod of cells, continuous 

 with the third groove of the head-kidney. It extends through 

 a very few sections, and terminates by a fine point of about two 

 cells, wedged in between the Wolffian duct and germinal epithe- 

 lium (described above, Nos. 7 10, series A, Plate 27). 



In an embryo slightly older than the above, such as that 

 from which series B was taken, but still belonging to our first 

 stage, a definite lumen appears in the anterior part of the 

 Miillerian duct, which vanishes after a few sections. The duct 

 terminates in a point which lies in a concavity of the wall of the 

 Wolffian duct (Plate 27, Nos. I and 2, series G). The limits of 

 the Wolffian wall and the pointed termination of the Miillerian 

 duct are in many instances quite distinct ; but the outline of the 

 Wolffian duct appears to be carried round the Miillerian duct, 

 and in some instances the terminal point of the Miillerian duct 

 seems almost to form an integral part of the wall of the Wolffian 

 duct. 



The second of our stages corresponds with that in which the 

 atrophy of the head-kidney is nearly complete (series D and H, 

 Plate 28). 



The Miillerian duct has by this stage made a very marked 

 progress in its growth towards the cloaca, and, in contradistinc- 

 tion to the earlier stage, a lumen is now continued close up to 

 the terminal point of the duct. In the two or three sections 

 before it ends it appears as a distinct oval mass of cells (No. 10, 

 series D, and No. I, series H), without a lumen, lying between 

 and touching the external wall of the Wolffian duct on the one 

 hand, and the germinal epithelium on the other. It may either 

 lie on the ventral side of the Wolffian duct (series D), or on the 

 outer side (series H), but in either case is opposite the maximum 

 thickening of that part of the germinal epithelium which always 

 accompanies the Miillerian duct in its backward growth. 



In the last section in which any trace of the Miillerian duct 

 can be made out (series D, No. 1 1, and series H, No. 2), it has no 

 longer an oval, well-defined contour, but appears to have com- 

 pletely fused with the wall of the Wolffian duct, which is accord- 

 ingly very thick, and occupies the space which in the previous 



