OF THE MULLERIAN DUCT OF AMPHIBIANS. 517 



runs down to lie opposite the segmental duct there is an important difference. The 

 epithelium shows no longer merely a thickening of two or three cells deep ; it is 

 differentiated to form a rod of cells (fig. 16). The segmental duct is still intact; its 

 outline can be followed at this stage as well as in any other form I have examined ; and 

 I find no sign that it participates in the formation of the rod. 



For the first few — three or four — sections after the closing of the second nephro- 

 stome there is merely a band of cells similar to what I have described in stage " a." 

 But further back the cells are arranged so that in the centre of the thickening a definite 

 rod is formed. The segmental duct is rounded and clearly defined, and shows no 

 appearance of dividing (fig. 16). After about ten sections, the rod ceases to be quite 

 distinct, and merges in the cell-mass, which gradually thins down, till it is lost in the 

 ordinary epithelium. 



The main conclusions that I draw from the above facts are, (1) that the anterior end 

 of the Miillerian duct is formed from the thickened epithelium plate that spreads from 

 the nephrostomes over the whole surface of the pronephros and beyond it ; and (2) 

 that the rod of cells that passes backwards along the segmental duct, and that later 

 becomes the Miillerian duct, arises from a prolongation of this epithelial plate. It 

 seems to me that in the specimen of Salamandra atra " B," with its still distinct 

 plates of heightened epithelium round the nephrostomes, and with its posterior process 

 of cylindric cells passing from the second nephrostome along the outside of the seg- 

 mental duct, we have a stage in the development of the Miillerian duct in which both 

 anterior and posterior are represented. 



Before proceeding to follow out the development of the duct I shall consider some 

 of the opinions that have already been expressed as to the first Anlage. 



Furbringer, the first to give a full account of the early stages of the Miillerian 

 duct of an Amphibian, found in Salamandra maculosa no appearances that are incon- 

 sistent with my view. In his stages " A " and " B," though he says there is no trace 

 of any distinct cord or canal which could be regarded as the Miillerian duct, he yet 

 describes an " inselartige Epithelerhohung, die mit dem Divertikelepithel Zusammen- 

 hangt." In his stage " C," while the right side is as in "A " and " B," on the left 

 side : " zeigt die Stelle des erhohten Epithels eine kleine Ausbuchtung, an die sich un- 

 mittelbar ein solider "04-0 "05 dicker Strang anschliesst der dem Vornierengang 

 ventral dicht anliegend naeh kurzem Verlaufe (wahrend vier Querschnitten) sich in die 

 ventrale Wand desselben verliert." This cord Furbringer recognises as the " erste 

 Anlage " of the Miillerian duct ; but he goes on to say : " Der damit gegebene Fund 

 lasst der Erklarung zwei Moglichkeiten offen : entweder ist der Miiller'sche Gang 

 entstanclen durch eine successive von vorn nach hinten schreitende Abschniirung von 

 der ventralen Wand des Vornierengangs oder er hat sich gebildet durch eine Wuche- 

 rung von dem erhohten Peritonealepithel aus nach hinten, worauf er schliesslich mit 

 der Wand des Vornierenganges verwachsen ist." He decides the difficulty by reference 

 to a later stage, in which he was unable to examine the posterior end of the cord, but 



VOL. XXXVIII. — PART III. (NO. 12). 3 Z 



