MORPHOLOGY OF CYCLOSTOMATA. 371 



of the prouepliic tubules which in attained by the embryo in 

 Stage III, is, therefore, six pairs. 



lo. The tubules of the first and second pairs come, in Stage 

 III, temporarily in close contact with the epiblast, but do not 

 receive cells from it ; they soon return to their original condition. 



16. The anterior extremity of the system shows, from the 

 first, degenerating features. The first, second, and sixth of the 

 tubules begin, during Stage iii, to decline ; and at the end of 

 Stage IV, or the beginning of Stage v, the tubules are reduced 

 into the minimum number, which consists of three pairs from 

 the third to the fifth. These three pairs function as the actual 

 excretory organ for a considerable length of time. 



17. Ketrogression is first met \vith in the first pair of tbe 

 tubules, which decline probably without further development, 

 soon after their separation from the myotome is completed ; they 

 seem to atrophy from the free end. The next pair degenerating 

 is the sixth, which is at first cut off from the collecting duct 

 and remains for a short time, but soon disappears without leaving 

 a trace. The second pair persists for some time seemingly to 

 function as the excretory organ, but it atrophies already in 

 the early part of Stage v, the communication wnth the coelomic 

 cavity being first obliterated ; and in Stage vi, none of the struc- 

 ture remains to be recognized. 



18. The foremost pair of the persistent tubules comes to 

 lie in close contact with the hind wall of the branchial chamber. 

 The two mesoblastic somites wdiich correspond to the first and 

 second nephromeres should therefore be looked upon as having 

 entered into the formation of the branchial region. 



The stages in which the tubules appear and abort in different 

 somites are shown in the annexed table. 



