10 F. M. BALFOUR. 



describing, in as matter-of-fact a way as possible, the actual 

 observations we have made, and then stating what conclusions 

 we think may be drawn from these observations. 



We have found it necessary to distinguish three stages in the 

 growth of the Miillerian duct. Our first stage embraces the 

 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, continu- 

 ous 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 

 epithelium (described above, No. 7 — 10, series a, Plate I). 



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 I, Nos. 1 and 2, series g). The limits 

 of the Wolffian wall and the pointed termination of the Miiller- 

 ian 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 II). 



The Miillerian duct has by this stage made a very marked 

 progress in its growth towards the cloaca, and, in contradistinction 

 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. 1, 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. 11, and series h. No. 3), it 

 has no longer an oval, "well-defined contour, but appears to 

 have completely fused with the wall of the Wolffian duct, which 

 is accordingly very thick, and occupies the space which in the 

 previous section was filled by its own wall and the Miillerian duct. 

 In the following section the thickening in the wall of the 



