GIG PR GREGG WILSON ON 



a second furrow appears, indenting the lateral wall of the ccelom, and as it gets deeper, 

 cutting oft* the Mullerian duct foundation from the lateral wall (fig. 6). Further back, 

 the band of thickened epithelium passes on to the mesonephros. Some twenty-five 

 sections — about 150 m — posterior to the passing over of the Anlage to the excretory 

 organ, the grooved thickening closes to form a tube. Through thirty sections more this 

 remains open ; then the Mullerian duct foundation abruptly ends, and its tip or growing- 

 point appears, independent alike of the contiguous ccelomic epithelium and of the sec- 

 mental duct that lies just internal to the Mullerian duct. The segmental duct, it is true, 

 shows an obliquity in a few of the sections near the termination of the Mullerian duct, 

 and the obliquely cut side nearest to the Mullerian duct Anlage might, at first sight, be 

 supposed to show a process of budding : but careful examination makes it clear that the 

 Mullerian and segmental ducts are well marked oft' from one another, and the staining 

 differences emphasise this distinction. The obliquity of the sections of the segmental 

 duct in the neighbourhood of the growing point of the Mullerian duct is to be explained 

 by the intrusion of the Mullerian duct causing the segmental duct to deviate from its 

 first position close to the ccelomic epithelium. 



Outside the Mullerian duct there is a distinct thickening of the ccelomic epithelium, 

 and this is specially noticeable in the region immediately posterior to the growing tip of 

 the duct ; but the thickening does not appear to take part in the formation of the duct 

 proper, though it may contribute to the formation of sheathing tissue. 



It thus appears that the simple epithelial thickening found in the region of the prone- 

 phros in specimen A is represented in B by a similar lateral plate with two posterior 

 continuations, the one ventral, and passing into the ordinary ccelomic epithelium, the 

 other dorsal, passing into the already tubular Mullerian duct. It is, of course, open to 

 anyone to assert that there may be a missing intermediate stage between A and B, show- 

 ing trace of the derivation of some of the cells of the Mullerian duct from the segmental 

 duct. I can only say that the anterior of the Mullerian duct is clearly derived from the 

 ccelomic epithelium, and that at such an early stage as is represented in B there is no 

 evidence of any other derivation; while growth backwards is distinctly not dependent 

 on any budding from the segmental duct. 



The two new facts that I am able to add from my examination of stages A and (5 to 

 what Wiedersheim has written on the subject are: — (1) That the anterior and first 

 foundations of the Mullerian ducts are thickened areas of the ccelomic epithelium in the 

 region of the pronephros ; and (2) that there is a distinct ventral development of this 

 foundation, comparable to the temporary ventral development of the Mullerian duct in 

 liana. 



Both of these facts are made clearer by the study of longitudinal horizontal sections. 

 Fig. 7 represents such a section through an embryo that is somewhat further advanced 

 than B. The open ostium abdomihale is seen, and stretching away in front of it, and 

 anterior to the mesonephros, is the thickened plate of epithelium that has already been 

 described as the first foundation of the Mullerian duct. Fig. 8 shows a more ventral 



