94 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



C. Hyla. 



After writing the above description of the development of the Mlll- 

 lerian duct in Eana sylvatica, I chanced upon a form, Hyla versicolor, 

 which seemed to show conditions worthy of careful study. As the 

 results tend to confirm the conception of the multiple origin of the Mtil- 

 lerian evagination in Rana sylvatica, which was expressed in the above 

 description, I have allowed that to stand as it was written. 



The peculiarity in the development of Hyla wliich makes the develop- 

 ment of the ostium more easily intelligible than that of Rana lies in the 

 fact that only the two posterior pronephric nephrostonies, instead of all 

 three, fuse to form a " common nephrostome." At what age the fusion 

 of the two nephrostomes takes place could not be determined, as it had 

 already taken place in my youngest specimen, in which the hind legs 

 were still quite inconspicuous. In this specimen the second and third 

 primary nephrostomes empty at some little distance from the peritoneal 

 surface into a ciliated common nephrostome. 



The development of the Miilleriau duct takes place at about the same 

 stage, relative to the external signs of metamorphosis, as in Rana syl- 

 vatica. In the youngest stage which I shall describe it is evident that 

 the pronephros has but recently ceased functioning, as degeneration of 

 the tubules and Wolffian duct has not proceeded far. Already, liowever, 

 those thickenings of the peritoneal epithelium which participate in the 

 formation of the Miillerian duct are conspicuous. Their condition is as 

 follows : near the anterior end of the body cavity a thickening of the 

 epithelium is visible ventrally. This curves dorsad and caudad, growing 

 thicker as it proceeds. When it reaches the first nephrostome it is con- 

 tinuous with a much more marked thickening surrounding that opening. 

 Conti-ary to the condition in Amblystoma and Rana, it is the dorsal lip 

 which is especially thickened, and, as will be seen later, it is this thick- 

 ening ievg. 1, Fig. 81, Plate 7) immediately dorsal to the nephrostome, 

 which forms the anterior Miillerian evagination. 



The first nephrostome opens anterior to the root of the glomus. The 

 second — the common nephrostome — opens posterior to it. Along a 

 line joining the first with the common nephrostome the general thicken- 

 ing of the surface of the pronephros is accentuated to such an extent that 

 there is formed a conspicuous band of large cells. This band is almost 

 invariably folded so as to form a groove or trough (Fig. 83, tae. e'th.), 

 whose concavity is directed toward the coelom. A thickened disk dor- 

 sal to the common nephrostome is also visible (Fig. 82, evg. S, 3). Pos- 



