82 bulletin: museum of comparative zoology. 



massive and sends a cord cephalad (the prc-coeloraic duct) a distance of 

 some fourteen sections.^ Just anterior to the evagination, this cord 

 shows, by the perfectly radial arrangement of its nuclei, that it is really 

 a potential duct. In passing forward this radial arrangement is soon 

 lost, but the cord remains very conspicuous and runs directly cephalad, 

 deeply buried in the pronephros. Figure 48 (Plate 4, dt. pr^coel.) shows 

 a cross-section of the cord five sections posterior to its cephalic end. 

 The arrangement of cells in the form of loose concentric layers is very 

 characteristic of this pre-coelomic duct, as well as of the iDosterior duct 

 of the anterior evagination and, to a less degree, of the duct from the 

 second or posterior evagination. 



The posterior duct of the anterior evagination extends back only a 

 few sections. I am convinced that this is not due to its never having 

 been formed, but to its early degeneration, for the duct from the second 

 evagination is in many places accompanied by a second duct, and finally 

 sends off a free cord, which runs parallel with it for some distance and 

 then ends. It would seem, then, that the duct from the first evagina- 

 tion had reached back to, and fused more or less completely with, the 

 duct from the second evagination, and that it had then almost wholly 

 degenerated between the two evaginations. I lay special stress on 

 the condition on this side of the body, because I think it explains a 

 puzzling condition on the opposite side. 



The first (anterior) evagination on the left side sends back a cord 

 which reaches the level of the second evagination and is there lost 

 sight of (compare Fig. L, p. 88). The second evagination sends back 

 its cord for about a somite and a half. Then for a distance there is no 

 sign of one, until suddenly it reappears again for a short distance. This 

 condition made me think, at first, that Wilson and others were right in 

 believing a part of the duct to be formed in situ from the cells of the 

 oviducal welt. I am convinced, however, from the condition on the 

 right side described above, that this isolated piece is simply a fragment 

 of the cord from iho first evagination which has escaped degeneration. 

 This belief is supported by the fact that similar isolated fragments of 

 the first duct may remain, even in much later stages, between tlie two 

 evaginations, where there can be no doubt as to their origin (see 

 dt, i, Fig. 52, Plate 4 ; and, for description, p. 87). 



To sum up the condition of the fundament of the Mtillerian duct at 

 this stage : The thickened disks ventral to the first and second nephro- 

 stomes have evaginated to form open pits, which have begun to migrate 



1 On the left side of the body it extends cephalad for 19 sections. 



