MUSEUM OF COMPARATIVE ZOOLOGY. 221 



ventral union of the walls of the pronephric cavity is, as I have shown, 

 artificial ; the constriction between the middle portion of the lumen and 

 the dorsal, or nephrostomal, portion is more apparent than real, for it is 

 formed by the posterior wall of the nephrostomal tube, the plane of the 

 section not having cut exactly in the axis of the tubule. In the section 

 following that shown in Figure 20, the pouch is detached from the peri- 

 toneum, and presents an appearance similar to that shown in Figure 19. 

 Before the third nephrostome is reached, the canal is divided by a hori- 

 zontal constriction into two tubes. The dorsal portion forms the tubule 

 of the third nephrostome ; the ventral portion is the anterior end of the 

 segmental duct. Figure 21 shows these parts in the region of the third 

 nephrostome. The section corresponds in position with the dotted line 

 21 in Figure 39. 



In the following sections the duct rapidly assumes a more dorsal posi- 

 tion (compare Fig. 39). It then proceeds directly backward, at the level 

 of the constriction between protovertebrse and lateral plate. Figure 22 

 shows the duct in the region of somite VI. It has not yet been formed, 

 however, throughout its entire length. On passing posteriorly, it grad- 

 ually loses its lumen ; then the circular arrangement of the nuclei indi- 

 cating the position of the lumen also vanishes ; the structure at length 

 terminates as a simple thickening of somatopleure in the region of the 

 tenth somite. In a few individuals, however, I found slight evidences 

 of a mode of ending different from that just described. In one case 

 the indications seemed so strong as to compel me to seek confirma- 

 tion of the view that the duct takes its origin in situ. I shall therefore 

 give the details of the evidence on this point, and discuss its probable 

 significance. 



Figure 23 represents in cross section the fundament of the duct in this 

 specimen, as shown in the fifth section in front of its termination. The 

 section of the mass here contains about eight cells, which are in close 

 contact with the somatopleure. In the second section behind this one 

 there are shown parts of four or five cells (Fig. 24). The protoplasmic 

 patch in the centre (rd.) is wider than an average cell of the fundament, 

 and probably represents the anterior ends of two cells lying in the fol- 

 lowing section (Fi?. 25, c. and d.). Dorsal to this mass of protoplasm is 

 a nucleated cell (b.), and above this a small area of protoplasm with a 

 faint nucleus («.) which is doubtless a portion of a cell the principal part 

 of which was cut off by the preceding section. On the ventral side of 

 the centre of the fundament there is also a round nucleated cell (<?.). In 

 the next following section (Fig. 25), there are two nucleated cells in the 



