THE DEVELOPMENT OF THE MULLEPIAN DUCTS OF REPTILES. 619 



The next stage is found in an embryo (C) of 25 mm. In this, as in B, the lateral 

 thickened area of the ccelomic epithelium is undermined ; and ventral and dorsal exten- 

 sions of it pass towards the posterior. Here, also, the ostium lies into the side of the 

 mesonephros. The funnel turns towards the posterior, and its lumen may be followed 

 back through several sections. Further back there is a solid rod of cells in direct con- 

 tinuation of the tube, but this extends only through a few sections. On the right side 

 the tip of the rod is quite distinct from the segmental duct, beside which, however, it 

 lies. On the other side I had, at first, difficulty in distinguishing the tip of the Miillerian 

 duct from the obliquely cut segmental duct, which, from being somewhat remote from 

 the ccelomic epithelium, passes close up to it, just behind the Miillerian duct. Fig. 16 

 shows the blunt point of the latter lying close to the segmental duct. Fig. 17 shows 

 the next section with no remainder of the Miillerian duct, but with the segmental duct 

 filling the space immediately posterior to the end of the Miillerian duct. Here, as in 

 other cases, we must assume that the growing tip of the Miillerian duct thrusts itself 

 between the segmental duct and the ccelomic epithelium, and causes a shifting of the 

 position of the former. Fig. 1 8 bears out this view by showing the altered position of 

 the segmental duct in the second section, posterior to the end of the Miillerian duct. It 

 shows, at the same time, the inward proliferation of the epithelial cells in this neighbour- 

 hood, described by Professor Wiedersheim ; but I would point out that these are 

 probably destined to take part in the formation of the sheath of the Miillerian duct, 

 rather than the duct itself; for the posterior end of the duct is absolutely distinct from 

 the thickened epithelial band (fig. 16). 



A further stage is met with in an embryo (D) of 23 mm., in which the Miillerian 

 ducts are considerably better developed. The relations of parts at the anterior end of 

 the foundation are similar to those described in C. The ostium abdominale lies, as in 

 other specimens, on the side of the mesonephros, at its anterior end ; the open funnel 

 passes inwards and back towards the posterior through five or six sections ; then a rod- 

 like continuation is met with in several sections, behind which a lumen is again met 

 with ; and after a few sections the foundation ends. In this specimen, again, the tip of 

 the Miillerian duct lies on the mesonephric duct. 



The last specimen (E) that I have to describe is of unknown length. It is consider- 

 ably larger than D, and shows the relations of the various parts more distinctly than any 

 other stage that I have. 



A few sections after the beginning of the excretory organ, the lateral diverticulum is 

 met with. Projecting into this from the side of the excretory organ is the thickened 

 epithelial plate, in which, further back, the ostium abdominale appears. Just outside the 

 excretory organ, on the lateral wall of the ccelom, there is a remnant of the original 

 anterior plate, and it projects towards the thickening on the excretory organ. In more 

 posterior sections the ventral thickening may be traced passing back, and extending far 

 round ventralwards. 



The ostium abdominale leads to a solid foundation, which is found through many 



