MUSEUM OF COMPARATIVE ZOOLOGY. 245 



Stage IV. 



Plate V. Fig. 43. 



Embryos of this stage measure from 2.8 to 3.1 mm. in total length. 

 Muscular fibres have begun to appear in the myotomes, the auditory 

 vesicles are entirely detached from the external ectoderm, and the pro- 

 tovertebrae have been differentiated as far back as the anus. 



The pronephric pouch of Bufo is very similar to that of Rana. It 

 communicates with the coolom by means of three nephrostomes, and 

 from its ventral margin the duct takes its origin. The nephrostomes 

 are segmental in position, and are situated beneath protovertebrae II., 

 III., and IV. 1 



The duct can be followed for some distance posterior to the hinder- 

 most pronephric nephrostome as a distinct elliptical tube with a central 

 lumen. The lumen, however, disappears further posteriorly, and the 

 duct terminates either as a simple thickening of the somatopleure, or its 

 posterior end merely rests upon the mesoderm in the region of somite 

 XI. The hinder tip of the duct (Fig. 43, /W. dl. sg.) in both cases re- 

 sembles very closely the adjacent mesoderm both m the size and in the 

 abundance of yolk spherules, and it differs from the ectoderm both in 

 these features and in the scarcity of pigment. In Bufo I have never 

 been so fortunate as to find the growing end of the duct situated in a 

 groove of depressed mesoderm ; but I believe that the fundament ex- 

 tends itself from the region of its origin in the somatopleure to the pro- 

 jecting cornu of the cloaca by means of an independent growth on the 

 part of its own cells. The greater part of the duct, however, arises from 

 a local proliferation of somatopleure. 



The pronephric capsule in Bufo arises as a downgrowth from the outer 

 peripheral layer of the protovertebrre. In this stage, however, it has not 

 reached the somatopleure ventral to the pronephros, but merely forms a 

 two-layered scale-like sheet of tissue covering the dorsal portion of the 

 gland. 



The pronephric chamber is present at this stage. The general bodv 

 cavity, however, has not yet appeared, the somatopleure and splanchno- 

 pleure being in other regions in close contact. 



1 I have preserved in the enumeration of the body somites of Bufo the same 

 designations that were employed in tlie case of Rana. In Bufo, however, the ker- 

 nel of the degenerate protovertebra in front of somite I. gives rise to a few muscle 

 fibres. 



