MUSEUM OF COMPARATIVE ZOOLOGY. 253 



portion of the latter which intervenes between nephrostomes I. and II. ; 

 and in discussing the topographical relations of the tubules it will be 

 needless to distinguish this remnant of the collecting trunk from the 

 first nephrostomal tubule. In this simplest condition of the pronephros, 

 the common trunk makes a single loop, the anterior curve of which is 

 situated nearly as far forward as the level of the first nephrostome. In 

 somewhat older pronephridia (Figs. 59, 60) the main bend of the common 

 trunk occupies a position even in front of the first nephrostome, and a 

 number of minor folds intervene between the junction of the nephrostomal 

 canals and this most anterior fold. In none of the pronephridia of this 

 stage is there any evidence of convolution in the nephrostomal canals. 



One individual of this stage departed from the normal condition, in that 

 it possessed three instead of two nephrostomal canals. This abnormality 

 occurred on both sides of the body, and appears to be correlated with a 

 less highly developed first nephrostomal tube. It is to be noted that 

 the third tubule (Fig. 57) appears as an appendage attached to the 

 most posterior loop of the common trunk. This topographical relation 

 suggests that it is the most posterior of the three nephrostomal tubules 

 which has been added to those normally present in Amblystoma, and 

 this inference is shown to be correct by the relations which the several 

 tubules bear to the body somites. The question whether the most 

 posterior of the three tubules in this case represents the third nephro- 

 stomal tubule of the Ann ran pronephros can be answered only b} r a con- 

 sideration of the relations which the several nephrostomes in the two 

 groups bear to the overlying protovertebrse, and will be recurred to in 

 the general discussion which follows. I may here anticipate to the extent 

 of stating that the first and second tubules of Amblystoma probably 

 correspond respectively to the second and third of Rana and Bufo, the 

 abnormal third tubule belonging to a yet mure posterior metamere. 



The position of the pronephric nephrostomes with reference to the 

 myotomes was determined at an early stage by the location of the first 

 metameric diverticula which are developed within the pronephric mass ; 

 and in the present stage these relations have not materially changed. 

 The two nephrostomes of the normal pronephros lie beneath the third 

 and fourth myotomes respectively. In the case of the pronephridia 

 with a supernumerary nephrostome, the first two nephrostomes occur 

 beneath myotomes III. and IV. respectively, while the third nephro- 

 stome is found beneath myotome V. 1 



1 Myotome I. of this enumeration reaches forward to the root of the vagus nerve, 

 and is flanked on its outer face by a portion of the ganglion nodosum, exactly as 

 in the case of the Anura described. 



