hall: mesonephros and mullerian duct in amphibia. 41 



and Stage IL Tlie changes will be seen to consist in an alteration in 

 shape and in an increased difference in the relative thickness of the 

 walls. In Figure 3 it will be noticed that the lateral wall is bent out- 

 ward to a slight extent just above the duct (at the point marked so. v-L). 

 In Stage III this bend is so pronounced that the junction of mesomer and 

 lateral plates (Figure 13, npli'tm.) lies ventral to the edge of the somite. 

 The ventral wall of the somite thus contains a small amount of somato- 

 derm. By the increased thickening of the anterior, posterior, and median 

 walls, the cavity of the somite has become nearly obliterated. There is 

 left only a narrow fissure-like lumen extending from the ventro-median 

 to the lateral and thence to the dorsal angle. 



One of the most important points to be noticed in this stage is the 

 beginning of the differentiation of the sclerotome, the first indication of 

 which may be seen in Figure 13, in the slight extension (scVtin.) of the 

 ventral wall of the somite toward the chorda. The tissue which forms 

 the sclerotome is certainly derived from the splanchnoderm of the ventral 

 wall of the somite, but at precisely what point or points the cell multi- 

 plication takes place, I have been unable to determine. The peculiar 

 form and arrangement of the cells of the median portion of this ventral 

 wall (see Figure 13) suggest, however, that this cell-layer is being 

 shoved mediad, and I suspect that the increase of cells takes place quite 

 near the point of union of mesomer and lateral plates, perhaps near the 

 point a. However this may be, I think we are justified in considering 

 all of the splanchnoderm of the ventral wall lateral to the point a, as 

 belonging to the mesomer. The mesomer thus consists of a splanchnic 

 portion, just described, extending mesiad from the point of union with 

 the laterad mesoderm at least as far as the point a, and a somatic por- 

 tion, which, beginning at the point of union with the lateral mesoderm, 

 extends laterad toward the outer angle of the somite (see the discussion 

 of the position of the pronephric thickening in the description of 

 Stage I). 



The AVolffian duct at Stage III has joined the cloaca opposite the 

 anterior end of somite 19. In the anterior half of the embryo it already 

 possesses a small lumen. 



Germ cells are conspicuous from the posterior end of somite 11 to the 

 posterior end of somite 15. They are most conspicuous intersegmentally, 

 but extend a short distance beneath the two adjacent somites. In the 

 last segment hi which they are seen (somite 15), they extend along the 

 entire somite. Altliough partially forced into an intersegmental position, 

 the germ-cell masses do not lose their connection with the mesomers. 



