MORPHOLOGY OF CYCLOSTOMATA. 355 



peritoneum and the collecting duct ; it is the posterior wall of the 

 tubule in figs. 99. Fig. 101 represents the section through the axial 

 plane of the third tubule, the nephrostomes of which are recog- 

 nized more clearly in the section behind it (fig. 102, nst.3). The 

 tubules of this pair are comparatively not long. The fourth pair 

 of the tubules and their nephrostomes are obvious in fig. 104 

 {pt.4 and nst.4) which represents the third section behind that of 

 fig. 102 ; the tubules much resemble those of the pair in front, 

 showing the same convolutions as these. It is a peculiarity of the 

 present stage that the aperture of the nephrostomes of the third 

 and the fourth pair is not so wide open as in the last stage or 

 as in more advanced stages Î It is always nearly closed and slit- 

 like, so that we can hardly trace the communication between 

 the lumen of the tubule and the body-cavity. 



Fig. 103 represents the section intervening between the sections 

 shown in figs. 102 and 104. On the right side, the collecting 

 duct alone, and on the left side, the duct together with a small 

 part of the fourth tublile, is shown. The peritoneal membrane 

 on the dorsal end of the body-cavity is folded far into that 

 cavity (fig. 103, bs,). This fold is traceable from the anterior part 

 of the third tubule to the hind part of the fourth (figs. 100-104, fe.). 

 The space enclosed in this fold communicates freely with both the 

 tract of the dorsal aorta under the chorda and the tract of the 

 anterior cardinal vein outside of the })j()nephros and contains a 

 number of mesenchymatous cells which probably wander in from 

 the tract of the aorta and the anterior cardinal vein. As sub- 

 sequent history shows, this structure constitutes the beginning of 

 the glomerulus of the pronephroi^. 



Figs. 105 and 106 represent two contiguous sections immedi- 

 ately posterior to the section shown in fig. 104. In fig. 105 we 



