MOEPIIOLOGY OF CYCLOSTOMAÏA. 410 



ward growtli formed after such a condition is passed, and its 

 homologue can not be found anywhere in Pctromyzon. I believe 

 that the same can be stated of Amphibia which is, to judge from 

 Field's account, very much like Petromyzon in this respect 

 (see p. 401). 



According to tax AVyhe and Rabl, the duct in Selachia 

 appears in the seventh to the tenth (in Rabl's sense) somites of 

 embryos with 34-3Ö somites and, when it is later connected with the 

 cloacal wall, the connection is found, — as can be inferred from 

 fig. 7/; of van Wy'HE, — in the thirty-eighth (forty-second of Rabl) 

 Rumpfsegment (or further backwards) of Fristiurus embryos with 

 80 (van Wy'he) to 87 (Rarl) mesoblastic somites. There is found 

 in f^elachia, therefore, a number of the mesoblastic somites in 

 the region back of the pronephros, which do not give rise either 

 to the pronephric tubules or to the segmental duct, and the duct 

 grows backwards, free from the mesoblast inside, during the 

 period in which the somites increase from 34-35 to 80-87, and 

 for the space reaching from the eleventh or twelfth to the forty- 

 second (in the sense of Rabl) somite. Such a considerable 

 prolongation of the duct during this period is not observed in 

 Petromyzon^* and in the two other groups of Craniota mentioned. 



And furthermore, it is questionable whether, during this 

 posterior growth, the duct in Selachia receives the constituent cells 

 from the epiblast along its whole length, as Rückert and tax 

 Wyhe believe ; or only at the point of the epiblast overlying the 

 hind end of the pronephros, with which the duct is connected, 

 and posteriorly to this point grows free from both the epiblast 



1) At about this stage (Stage iv), there is no space left behind the pronephric system 

 segmentally formed; for the embryo ot Petromyzon, is retort-shaped and lias the anus situated 

 in the ventral median line of tlie bulb of the retort. 



