MORPHOLOGY OF CYCLOSTO:\rATA. 391 



and already in his Stad. in, a space is seen between these two 

 structures. 



The deg-eneration of the tuhules in Sehichia runs a course 

 parallel with that mentioned under Am2:)hioxii.s and C^'clostomata. 

 As seen above, tlie Anlagen of the pronephros are developed most 

 vigorously in tlie middle part of the pronephros, as in the case 

 of A'ii}pkwxu>^ and Cyclostomata ; and degeneration begins at the 

 cranial and caudal extremities as there. 



Vax Wyhe says that the degeneration consists in a confluence 

 (Verschmelzung) of the ostia. According to Ruckekt, the 

 Vornierenfalte becomes simply flattened out in the cranial part 

 of the pronephros. The reduction in the caudal part is noteworthy : 

 the Anlao-en are here constricted ofl" from the mesoblast and con- 

 verted into the anteriormost section of the segmental duct. 

 Only the middle (the third) diverticulum (in Torpedo) persists 

 in communicating with the bod^^-cavity and becomes the ostium 

 ahdominale. 



In Petromyzon, I have unfortunately failed to observe accu- 

 rately the manner of degeneration of the tubule in the cranial 

 part. It is however probable that it begins either from the 

 blind tip of the tubule (the first tubule), or by obliteration 

 of the nephrostome (the second tubule). In the caudal part, the 

 collecting duct is constricted off from the lateral plate by 

 obliteration of the tubule and constitutes the foremost section of 

 the segmental duct, in precisely the same manner as in Selachia. 

 The difierence is : in Petromyzon the communication with the 

 body-cavity is retained l»y the three middle nephrostomes, while 

 in Selachia, it is through only the middle one, that is, the ostlunt 

 ahdominale. 



The segmental duct becomes apparent in an embryo with 



