246 BULLETIN OF THE 



Stage V. 



Plate V. Figs. 42, 46. Plate VI. Figs. 47, 49, 52. 



At this stage the larvae were hatched and swam about freely in the 

 aquaria. The larvae measured from 4 to G mm. in length, and each had 

 a distinct tail, which protruded for a distance of 1.5 to 2 mm. behind 

 the anus. The pronephros was probably already functional. 



The character of the convolutions of the pronephric tubules was 

 studied in the case of four pronephridia. In this feature one of them 

 corresponded very closely with the condition in the pronephros of Rana 

 represented in Figure 33. The remaining pronephridia differed from 

 this type solely in the circumstance that the third nephrostomal tubule 

 joined the collecting trunk at t he extreme posterior portion of the bend, 

 which m Rana usually forms the first portion of the common trunk. 



The position of the pronephros with reference to the somites remains 

 in general nearly the same as in the preceding stage. In individual 

 cases, however, the nephrostomes do not appear to lie precisely under the 

 middle of the myotome. 



In embryos of this stage, the segmental ducts already open into the 

 cloaca. These openings are situated beneath myotome XII. It is 

 obvious from this fact that the duct in the older embrj'os of Stage IV. 

 had already very nearly reached the region of its final communication 

 with the cloaca. In Bufo the lumen of the gut is very narrow, and is sep- 

 arated from the lateral walls of the body by an extensive mass of yolk cells. 

 The cloacal cornua are therefore in this case very long, extending to the 

 outer surface of the entoderm. The ducts reach these cornua by passing 

 between the dorsal angle of the body cavity and the overlying myotomes. 



The histology of the pronephros in Bufo does not present any note- 

 worthy features of difference from that in liana. The tubes are all 

 slightly smaller in Bufo, and their walls contain somewhat more pigment 

 than do those of Rana. 



The capsule envelops the pronephros and duct in the way that I have 

 described for Rana, and it also encloses a series of blood sinuses which 

 are developed from the posterior cardinal vein. I was not able to obtain 

 in Bufo any additional evidence in regard to the origin of the mesenchyme 

 of the pronephros. 



Two veins emerge from the anterior end of the pronephros. One of 

 these is the immediate continuation of the posterior cardinal vein, which, 

 in passing forward as the ductus Cuvieri (Plate V. Fig. 42, dt. Cuv.), 

 makes a rapid ventral descent to open into the sinus venosus. The 



