228 BULLETIN OF THE 



is the growth of the collectmg trunk hi the regiou between the second 

 and third nephrostomal tubules, and the consequent separation of the 

 latter. The further complication in this case is mainly due to a con- 

 volution of the second tubule; slighter contortions occur in other parts. 

 In the case of the pronephros diagrammatically represented in Figure 31, 

 however, a canal, which corresponds to what we should regard in Figure 

 32 as the anterior portion of the segmental duct, has been folded first 

 forwards, reaching nearly to the level of the hrst nephrostome, and then 

 backwards. The bends which are convex anteriorly may be called the 

 anterior bends-, those which are convex posteriorly, the posterior bends. 

 The universal occurrence of this condition in all older embryos makes 

 it desirable to distinguish this bent portion of the tube and its deriva- 

 tives both from the original longitudinal canal of the pronephros, which 

 I have called the collecting trunk, and from the straight posterior por- 

 tion, or segmental duct proper. In the following pages I shall speak 

 of each nephrosto^nal tubule as extending from its origin in the nephro- 

 stome to its junction with -the longitudinal canal, or collecting trunk. 

 In the case of the first nephrostomal tubule, the point of union with the 

 collecting trunk is usually marked by an abrupt change of direction ; 

 where this does not occur, however, the distinction between the two 

 portions must be somewhat arbitrary. The collecting trunk forms the 

 continuation of the first nephrostomal tubule, it receives in its backward 

 course the second tubule, and may be regarded as terminating at the 

 point of entrance of the third tubule. The common trunk arises from 

 the point of junction of the third tubule with the collecting trunk, and, 

 after making various convolutions, leaves the gland at Its posterior end 

 as the segmental duct. In the two pronephridia shown in Figures 31 and 

 33, we have before us examples respectively of the two principal forms 

 of convolution which are to be recognized in subsequent stages, viz. the 

 contortion of the second tubule and that of the common trunk. The 

 third tubule finally undergoes convolution to some extent ; but the first 

 tubule and the collecting trunk take almost no part in the process. 

 Although complication has appeared both in the second tubule and in 

 the common trunk, it is to be noticed that these processes do not have a 

 fixed sequence. I have numbered the diagrams on Plate IV. with refer- 

 ence to the state of development shown by the larvae. In doing this, I 

 have not been guided by the age alone, for the large amount of indi- 

 vidual variation makes that method nearly valueless ; but I have en- 

 deavored, by passing in review a large number of characters, to gain a 

 notion of the relative degree of development shown by the larvfe. 



