238 BULLETIN OF THE 



distinguish : (1) posterior bend adjacent to the collecting trunk, (2) as- 

 cending arm, (3) anterior bend, and (4) the descending arm, which is 

 continuous with the segmental duct. The simplest condition which I 

 have found in Stage VI. is represented in Figure 35. This diagram 

 relates to a larva of R. sylvatica Le C, and it is of interest to note its 

 close similarity to Figure 38, which represents the pronephros of a 

 larva of II. pipiens Schreb. (halecina) of this stage. These two pro- 

 nephridia will be considered together. In both, the ascending arm of 

 the common trunk makes either an S-shaped bend or a loop interpo- 

 lated near the middle of its course ; the transverse portion, or anterior 

 bend, is thrown into one or two slight fulds, and the descending arm 

 shows two loops, one in the middle of the gland, and the other near 

 its posterior end. The two remaining diagrams (Figs. 3G, 37), though 

 taken from different individuals, are alike in all essential particulars. 

 The principal changes from the condition shown in the simpler prone- 

 phridia just described consist in the development of an additional loop 

 in the course of the ascending limb, and of several slight folds in the 

 transverse portion ; the loops present in the younger individuals of this 

 stage have persisted and become more extensive. In the case of the 

 larva whose pronephros is represented in Figure 37, I made a compar- 

 ative study of the pronephridia found on the two sides of the body. 

 The comparison showed that a slight want of symmetry existed between 

 the two sides. Occasionally the direction in which equivalent tubes 

 were bent did not correspond. On the right side of the body (the one 

 figured), for example, the hindermost loop of the descending arm was 

 formed by an inward bend, while in the left pronephros the corresponding 

 tube is bent outward. In the descending arm of the left pronephros a 

 small loop occurs in addition to those present on the right, while one of 

 the two loops occurring in the ascending portion of the right side is 

 almost unrepresented on the left ; thus, the right pronephros approxi- 

 mates in this respect the simpler organ represented in Figure 35. A 

 more striking anomaly of the left pronephros consists in the occurrence 

 of a slight bend of the collecting trunk between the junctions of the sec- 

 ond and third nephrostomal tubules, so that the latter connects with an 

 ascending portion of the collecting trunk. Finally, the third nephro- 

 stomal tubule of the left side joins the collecting trunk farther posteri- 

 orly than does the one on the right side. In general, however, it seems 

 to me that the several pronephridia studied show a rather remarkable 

 uniformity even in the details of the arrangement of their tubules. 



The position of the pronephros with reference to the muscle plates is 



