HALL : MESONEPHEOS AND MULLERIAN DUCT IN AMPHIBIA. 83 



caudad, especially the posterior one. From the anterior one a thickened 

 epithelial band runs caudad, then ventrad around the free end of the 

 shelf (which now extends some distance back of the pronephros), and 

 then forward on its ventral surface. Both on the ventral and on the 

 dorsal side of the shelf the band shows a tendency to slip off, as it were, 

 on to the lateral body-wall, so that in its backward course it now passes 

 close to the second evagination, thus reducing the length of the band 

 which connects it with tlie second evagination. 



From both first and second evaginations ducts (or cords) run caudad. 

 That from the first fuses more or less completely with that from the 

 second, and shows a tendency to degenerate between the two evagina- 

 tions. There is an additional cord extending cephalad from the first 

 evagination, which has no known ontogenetic meaning, but may have an 

 important phylogenetic significance. As this cord (which is sometimes 

 met with in later stages) completely degenerates without participating 

 in the formation of the Miillerian duct, I shall leave it out of the descrip- 

 tion of subsequent stages. Finally, the sub-gloraerular cavity has begun 

 to extend itself cephalad beyond the glomerular cavity. 



Larva VIII, 55 mm. 



In this and later stages the distortion suffered by the degenerating 

 pronephros gives rise to much variation in the relative position of its 

 parts. Sometimes, as in the present larva, the glomus, narrowly con- 

 fined in its cavity, lies wholly anterior to the tubule-mass of the 

 pronephros. It may, however, lie opposite, or almost wholly behind 

 the pronephros. Some or all of the nephrostomes may remain open 

 until a much later period. Generally, however, the anterior ones at 

 least are closed, and may even be wholly severed from the peritoneal 

 epithelium. A like variation is found, in their position. In general 

 there is a tendency toward a caudal migration. In one case the first 

 nephroatomal tubule on one side of the bodj' had in its migration passed 

 the second nephrostome and ended posterior to it near the peritoneum. 

 The nephrostomal cavities are usually absent. 



The condition of the evaginations in this larva differs from that in 

 Larva VII, only in that they have all migrated caudad (Fig. M, p. 88). 

 An important result of this migration is that it has brought the poste- 

 rior evagination into a position posterior to the free edge of the shelf. 



With the dorso-ventral compression of tlio pronephros (probably due 

 to the growth of the stomach), the anterior evagination no longer lies at 



