132 ]v\cretory Organs in Brlellnstoma Stouti 



posterior segments. But in younger embryos the dnct has since been 

 found in sections wliere the splanchnocoel had not yet been formed. 



In the anterior region the duct is occasionally either partially or 

 entirely absent between two tubules. Tliis is to be looked upon as due 

 either to degeneration or to the failure of the duct to develop. In an 

 extreme case, in an embryo of stage B, tliere was a Ijreak on one side 

 between tubules one and two, five and six, seven and eight, eight and nine, 

 and fourteen and fifteen, and on the otlier between one and two, five and 

 six, and nine and ten. 



We now come to the differentiation of the pronephros of the adult, 

 the most important steps of which occur in the stages between B and 0, 

 and were therefore not seen at all in the previous work. 



In the oldest embryo of stage B the anterior end of the body was 

 injured, so that the position of the anterior end of the excretory organs 

 could not be accurately determined, but it was estimated that the system 

 had disappeared in about nine segments. It is true that some of the 

 anterior tubules may degenerate, but in no case in the material in hand, 

 where the point could be accurately determined, can it be said that so 

 many as nine have disappeared, so that the above estimate must be 

 considered erroneous. What was said regarding the pronephros in em- 

 bryo C was of rather a tentative nature, the preservation not being of 

 the best, and it being impossible to work out the structure satisfactorily. 



In the youngest embryo before studied the gill-slits were found Avell 

 forward in the head region, while in the oldest they were well back in 

 the body region, occupying the same position as in the adult. On en- 

 tirely insufficient evidence it was assumed that the change had been 

 brought about by slits degenerating at the anterior end and new ones 

 being added at the posterior end, and that, in the course of development 

 a good many more slits appeared than were present in the adult. 

 Dean ^ has shown that this is wrong, the first slits formed being perma- 

 nent, but shifting their position from the head region to the body region. 

 This change in the position of the gill-slits seems to be the cause of an 

 important change in the excretory system. At all events, as the gills 

 become shifted farther and farther backward the anterior tubules be- 

 come crowded closer and closer together, until finally they form a small, 

 compact body, the pronephros of the adult, in the region a little posterior 

 to the thirtieth segment. The crowding affects all of the open tubules, 



"Dean, Bashford: On the Development of the California Hagfish, Bdel- 

 lostoma Stouti, Lockington. (Preliminary Note) Quart. Journ. Micr. Sci,. 

 No. 158. Vol. 40, 1897. 



