86 BULLETIN: MUSEUM OF COMPxVRATIVE ZOOLOGY. 



and 59 the mass is seen to have again attached itself to the Wolffian 

 duct and partially (Fig. 59) to the Mtillerian duct as well. The two 

 following sections (Figs. 60, 61) are especially significant, in that they 

 show a cell which has just divided into two daughter-cells, one of which 

 (Fig. 60) seems to constitute a part of the wall of the Wolffian duct, the 

 other (Fig. 61) a part of the cell-mass which in turn is quite intimately 

 connected with the Mtillerian duct. In the two following sections 

 (Figs. 62 and 63) the cell-mass nearly disappears, a portion of it allying 

 itself with the Wolffian duct, the remainder forming a separate strand 

 closely applied to the Mtillerian duct. This and the Miillerian duct now 

 become separated from the Wolffian duct by a layer of connective tissue. 

 The strand just described is more or less distinctly differentiated from 

 the mass of the Miillerian duct for some ten additional sections. It 

 then separates entirely from that duct and, passing again through a gap 

 in the connective tissue layer which separates the two ducts, fuses with 

 the Wolffian duct. The posterior end of this fusion is shown in Figure 

 64, where the lumen of the Wolffian duct is seen to present a slight 

 outpocketing ventrally, which represents the posterior end of the lumen 

 of the strand just described. Posterior to this point, the Mtillerian duct 

 extends free for some distance and then ends. 



The pictures presented by these sections may be interpreted in two 

 ways : Either the duct had fused with the W^olffian duct and is now 

 drawing away from it, leaving strands of tissue still connected with both 

 ducts, or the Wolffian duct is participating in the formation of the 

 Miillerian duct by proliferating cells which form irregular masses and 

 strands of tissue extending from the former to the latter. The second 

 view is supported by the presence of the dividing cell shown in Figures 

 60 and 61. 



My conviction that the Wolffian duct participates, to a small extent, in 

 the formation of the Miillerian duct, rests not only on the fact that at 

 certain stages there is always at least a juxtaposition of the two, but also 

 on the additional fact that, where this occurs, cell-division is frequently 

 taking place in the wall of the Wolffian duct. This formation of new 

 cells at a time and place where the Wolffian duct is functionless and 

 about to degenerate is surely very significant. 



Larva XI, ^9 mm. — No gills. 



The most anterior sign of the fundament of the Mtillerian duct in this 

 larva is the thickened band, which begins anterior to the pronephros and 

 extends caudad on the dorso-lateral wall of the sub-glomerular cavity 



