114 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The portion of the glomerular cavity surrounding the mouth of the 

 anterior of the two pits and the neighboring nephrostome becomes 

 partially closed off to form what I term tlie nephrostomal cavity. 



From the distal (deep) end of the first pit, or Miillerian evagiuation, 

 two ducts take their origin, one of which, the precoelomic duct, extends 

 cephalad and later degenerates completely. The other, which I have 

 called the first duct, ignoring the precoelomic duct, extends caudad past 

 the second evagination and there fuses with a similar (second) duct from 

 that evagination. That portion of tlie first duct which lies anterior to 

 the union with the second duct degenerates. 



In the mean time both evaginations have migrated caudad. This 

 movement withdraws the first evagination from the nephrostomal cavity 

 (which closes and with its associated nephrostome disappears) and brings 

 the second to lie behind the posterior limit of the shelf, and hence out 

 of the glomerular cavity. The thickened epithelial band is consequently 

 no longer forced to make a detour to reach the second evagination ; it 

 extends from the anterior end of the sub-glomerular cavity, back along 

 the ventral surface of the shelf (or along the adjoining lateral wall of 

 the body), and is continuous tlirough the second evagination, with the 

 duct leading caudad from the latter. 



The band from the first to the second evagination disappears. The 

 first evagination also degenerates completely. 



That part of the Miillerian duct which is immediately posterior to 

 the second evagination is composed of the duct of that evagination ^^Zms 

 a portion of the duct of the anterior evagination. After the fusion of 

 these two components to form a single duct, the latter grows back near 

 the Wolffian duct, fuses with it for a variable, but never great distance, 

 and then grows backward free. Later, the fused portion again sepai-ates 

 from tlie Wolffian duct. That the Wolffian duct contributes cells help- 

 ing to form the Miillerian duct in this region, seems almost beyond 

 question. The greater part of the duct, however, — that is, throughout 

 the entire extent of the mesonephros, — gi'ows back independent of the 

 Wolffian duct. 



The ^liillerian duct increases in length cephalad by utilizing the 

 thickened band which extends anterior to the second evagination, that 

 being the only one which persists. This is accomplished by a folding 

 of the band into a groove, beginning at the evagination and progressing 

 cephalad. Tlie fusion of the edges of the groove carries the open- 

 ing of the duct cephalad and ventrad to its position in the adult. 



