Development of tlie Kxiretory Or^cmis of Hdellostoma stouti Loi'kiiit^'toii. 211 



(Fig. 7i) the luineii from the tubule will be seen to have grown back 

 into the anläge of the duct. 



From here back to the sixty-second segment there is no essential 

 change in the tubules, as a glance at the tubule in Fig. 1, which is 

 in the twenty-seventh segment, and Fig. 8d, which is in the thirty- 

 fifth segment, will show. In the sixty-second segment, which is eighteen 

 segments in front of the posterior end of the segmental duct, the 

 tubule has lost its connection with the general body-cavity, and the 

 same is true for the most, though not for all, of the remaining tu- 

 bules. The process by which this is effected will be described later on. 



The constricting off of the segmental duct becomes more and more 

 marked, until in the region of the twenty-fourth segment the process 

 is complete, and the duct retains its connection with the coelomic 

 epithelium only by means of the tubules. This is true for the entire 

 remaining i)ortion of the duct on one side, but on the other there is 

 an interesting exception ; in the region corresponding to the posterior 

 mesonephric tubules of the adult, there are places where the duct is 

 connected with the coelomic epithelium for nearly the entire distance 

 between two tubules. In one case, for example, there are six sections 

 occurring between two tubules, and in only one of these is the duct 

 completely constricted off. This is important, as showing that here 

 the duct is formed in the same way as in the anterior region. 



The posterior end of the duct has no lumen, and it lies sur- 

 rounded by dense mesenchyme, so that its limits are not easily deter- 

 mined. It is certain, however, that it extends as far caudalwards as 

 in the oldest embryo, or in the adult, that is to the eightieth segment, 

 but it has not fused with the entoblast. In the last two segments 

 there are no tubules, and it is a question how the duct is formed 

 here. The fact that in some cases it extends seven or eight sections 

 beyond the posterior end of the body-cavity would seem to prove that 

 it had grown back independently. 



In Fig. 7i it was seen that the lumen of the tubule had begun 

 to grow back into the anläge of the duct. In the more posterior 

 region this process is carried still further, and in some cases the 

 lumen extends back through as much as half the distance between 

 two tubules, but there is no case where it extends through the entire 

 distance. Figs. 8d— 8g is a case illustrating this process. In some 

 cases the lumen grows forwards as well as backwards, and occasionally 

 a section is met with in which the lumen has formed independently, 

 but both of these are the exception and not the rule. 



