HALL: MESONEPHROS AND MULLERIAN DUCT IN AMPHIBIA. 67 



chamber. In this partial division, the anterior and posterior prolonga- 

 tions become associated exclusively with the dorsal chaoiber, which thus 

 takes on the form of a longitudinal tube whose ends are, at least gener- 

 ally, in close contact with tlie similar tubes (that is, upper chambers) of 

 the adjacent units. This condition will V)e made clear by a glance at 

 Figure 30 (Plate 3), wliich is a dorso-median view of a reconstruction 

 in wax of four blastulae and the Wolffian duct from the right side of a 

 larva. The two anterior blastulae are approximately at the stage repre- 

 sented in Figure 94, the third in tiiat represented in Figure 95. The 

 ventral chamber is destined to form the j)rimary mesonepliric unit. 

 The dorsal chamber remauis without change for a considerable period 

 and gives rise successively to the dorsal sets (secondar}^ tertiary, etc.) 



The primary blastula (lower chamber) enlarges, its median and lateral 

 walls showing a decided difference in thickness. The lateral wall, wliich 

 is the thicker, buds out an evagination, which abuts on the duct. This 

 stage is represented in Figure 96 (Plate 8), and in the most posterior 

 unit of the wax model (Fig. 30, Plate 3). A section corresponding in 

 position to the dotted line of Figure 30 is shown in Figure 28. 



At the next stage (Fig. 97, Plate 8) we recognize that the evagination 

 in Figure 28 is the fundament of the inner tubule. It has grown in 

 length, its free end (thl. ms'nph.) has been crowded upward, and there 

 it becomes attached to the duct. Ventro-median to the fundament of 

 the inner tubule, the wall of the blastula again evaginates atid prolif- 

 erates cells to form a cone extending toward the peritoneum (Fig. 97, 

 thl.nph'stm.). Tliis is, of course, the young outer tubule. At a stage 

 as early as that of Figure 97 the fundaments of all the parts of 

 the complete mesonephric unit can be recognized ; the inner tubule, the 

 outer tubule, and the glomerulus (exclusive of the vascular part), the 

 last consisting of the glomerular covering {cjis. Bow. i.) and Bowman's 

 capsule {('ps. Bow. ex.). Having determined the positions of the various 

 fundaments in the present stage, we can now identify their positions in 

 the preceding stage (Fig. 28), if we bear in mind that between these 

 two stages the unit has rotated some forty degrees about its longi- 

 tudinal axis (compare Figs. 96 and 97). In Figure 28 the fundament 

 of the outer tubule (thl. 7ipJi^stin.) composes the ventral wall of the 

 blastula median to the evagination which forms the inner tubule (fbl. 

 ms^npJi.). The fundament of Bowman's capsule is contained in the 

 tissue of the thin median wall (cps. Boio. ex.), and the glomerular cover- 

 ing is derived from the dorso-lateral portion of the thick lateral wall 

 {ci:)S. Boiv. i.). 



