13-i Excretory Oi'g.ins in Bdellostonia Stouti 



representeJ tubules. All arc connected with the segmental duct. If 

 they were arranged segmentally the first would be in the thirteenth seg- 

 ment. This embryo was sectioned transversely, and it could be deter- 

 mined accurately that the tubules in the thirtieth segment were the first 

 to be closed off. However, those in the twenty-ninth segment looked as if 

 they might be closed off later. 



The first tubnlc in the next embryo is in the twenty-fourth segment. 

 It is not connected with the rest of the system and is separated by quite 

 a wide space from the second, which is in the twenty-fifth segment. In 

 segments twent5'^-five to thirty there are sixteen tubules, and of these the 

 first ten, which are in the space of three segments, are so closely crowded 

 as to be almost if not quite in contact with one another. If all were 

 arranged segmentally the first tubule would here be in the fourteenth 

 segment. 



In the next embryo there are seventeen tubules in the four segments 

 twenty-seven to thirty. Of these, the first thirteen are in so short a 

 space that there is not room for them to stand in a row one behind the 

 other, and some are beginning to be crowded to one side — a process 

 which is carried much farther in older enibryos. A comparison of Fig. 

 28, taken from this embryo, with Fig. 27, shows how much closer together 

 the tubules are here than in the younger embryos. The coelomic opening 

 of most of the tubules shown in Fig. 28 are found in other sections. 

 The segmental duct is no longer straight, and so does not show in the 

 full length of the figure. 



The next embryo is particularly interesting, because here the crowding 

 process has affected not only all of the open tubules, but the three 

 anterior closed tubules as well. The posterior limit of what will be the 

 pronephros of the adult may be placed, with some degree of probability, 

 between the third and fourth closed tubules, for at this place the seg- 

 mental duct shows some slight indications of degeneration. Owing to 

 an injury in the anterior region of the body, it is not possible to deter- 

 mine the exact position of the pronephros, but it occupies the space of 

 a little less than two segments. There are in all twenty-one tubules_, 

 eighteen open and three closed. The first and second tubules are con- 

 nected with each other, but not with the rest of the system. The three 

 closed tubules have the same structure as those farther back, and are in 

 direct line with them, but they occupy the space of only three-fifths ol 

 fi segment, and the third slightly overlaps the second. Just at the point 

 where the anterior closed tubule joins the duct, the latter bends down- 

 ward, backward and slightly outward, and then turns and runs again 

 forward, thus forming a sort of s-shaped bend. In this way some of the 



