MUSEUM OF COMPARATIVE ZOOLOGY. 229 



The first of the series of diagrams (Fig. 31) shows complication to 

 have taken place to a considerable extent in both the convoluted regions. 

 In the next diagram (Fig. 32) the second tubule alone takes part in the 

 complication. Figures 33 and 34 represent respectively the right and 

 left pronephridia of one individual. In the right pronephros (Fig. 33) 

 the typical condition of the common trunk is present, while the neph- 

 rostomal tubules have undergone no contortion. Likewise in the left 

 pronephros (Fig. 34) it is the common trunk to which the increasing 

 complication is due ; but in this case there are two additional bends 

 introduced by a slight folding backward of the middle of the anterior 

 bend. The convolutions of the common trunk lie principally in the 

 ventral portion of the gland. The tubes which in cross section are seen 

 in the dorsal part are mainly the several nephrostomal tubules, and the 

 collecting trunk. This condition is likewise retained in later stages. 



The position of the pronephros with reference to the myotomes has 

 not changed since the preceding stage. The whole structure is slightly 

 longer, but the myotomes have also lengthened to the same extent. 

 The thi*ee nephrostomes are situated, as before, beneath the first, second, 

 and third myotomes posterior to the ganglion nodosum, and are seg- 

 mental in position. 



In all the embryos of this stage the duct has opened into the cloaca. 

 It is to be remembered in this connection, that the morphological posi- 

 tion of the duct is outside the somatopleure ; so that the ccelom and 

 two layers of mesoderm intervene between it and the intestine. As 

 might be expected, the union does not take place until the segmented 

 and unsegmented portions of the mesoderm have become separated 

 from each other. The passage to the cloaca is then effected through 

 the split thus produced, and consequently around the dorsal angle of 

 the body cavity. 



In the frog, there is a sharp histological contrast between ectoderm 

 and entoderm, and there is therefore no difficulty in assigning a limit 

 to the proctodreal invagination. The region into which the duct opens 

 is the hind gut, and the intestine at this point is unquestionably lined 

 with entodermal cells. The portion of the primitive gut posterior to the 

 openings of the segmental duct forms the Amphibian cloaca, and corre- 

 sponds precisely, I should say, with that part of the cloaca of Amniota 

 which Gadow ('88, p. 28) has recently designated by the name urodaeum. 

 The wall of the intestine is not wholly passive in the union occurring 

 between it and the duct. In front of the excretory openings, the lumen 

 of the intestine has an elliptical form, its major axis being vertical. 



