MORPHOLOGY OF CYCLOSTOMATA. 343 



behind the section sliown in fig. 70 ; the tubules of the fourth 

 pair {pt.4) show themselves symmetrically on both sides; they 

 are somewhat less developed as compared with those of the last pair. 

 Fig. 72 is the section next behind fig. 71 ; it shows on both sides 

 tlie collecting duct {cd.) together with the coelomic projection {c.j}.) 

 which is a part of that of the anterior segment. Fig. 73 is the third 

 section posterior to that just described ; the blastoderm becomes 

 more flattened than in the foregoing sections ; it shows on both sides 

 the tubules of the fifth pair (pt.ô) ; the condition of the tubules 

 and nephrostomes {nd.ô) is much like that in fig. 71. Fig. 74 

 is from the fourth section posterior to fig. 73 ; the right tubule 

 {pt-6) of the sixth pair and its nephrostome {nst.6) are visible on 

 the right side, while the collecting duct {cd.) is cut through on the 

 left. The sixth tubule and nephrostome on the opposite side are 

 observed in the next anterior section which is not figured. The 

 segments back of the ninth somite have no trace of the tubule, but 

 the cross-sections of the posterior continuation of the collecting- 

 duct, the segmental duct, are repeated in each section. Fig. 75 

 represents a section through the sixteenth somite ; the cross- 

 section of the segmental duct {sd.) on either side is seen ; it 

 always occupies the space where, in the anterior region, the 

 tubules or the collecting duct is found. 



This condition, however, is not continued to the dorsal lip 

 of the blastopore. As I have stated in my previous paper ('91), 

 many processes of development are much delayed in the hind region, 

 so that we are here reminded of wdiat were seen in the anterior 

 region of the younger stages. Fig. 76 represents the fifteenth sec- 

 tion from the dorsal lip of the blastopore and passes through about 

 the twenty-third somite. In this comparatively late stage, in 

 which many mesoblastic organs have developed in the anterior 



