392 s. HATTA : 



35 (van Wyhe), or 34 to 3ö (Rabl) somites. The anterior 

 small section of tlie duct is formed, as just stated, in the same 

 manner in Petromyzon and Sekichia. The mode of formation of its 

 posterior hirger portion in Selachia differs from that of Petromyzon. 

 RÜCKERT ('88) and van Wyhe ('88, '89, '98) believe that 

 it is the product of the epiblast^\ while Eabl maintains its 

 purely mesoblastic origin. At any rate, the posterior tip of the 

 duct or the cord is sharply pointed and connected firmly 

 with the epiblast throughout its growth until it opens into the 

 cloacal cavity, which is effected, according to van Wyhe and 

 Eabl, in the embryo with 83 to 84 somites. It can be inferred 

 from VAN Wyhe's figs la and lb, that this communication is found 

 in a plane vertical to the thirty-eighth Eumpfsegment'-^ In 

 Petromyzon, the duct, being formed of a series of abortive pronephric 

 tubules, has no genetic relation to the epiblast except in the 

 cloacal region where the duct seems actually to receive cells from 

 the epiblast, as fully stated above (p. 366). 



The nephric arteries of Selachia which were discovered by 

 Paul Mayer without reference to their relation to the pronephros, 

 were studied by Euckert and their true nature was pointed out 

 by him. There are six of them in Torpedo corresponding to the 

 number of the nephric segment ; they are, however, not somitic 

 but intersoinitic in position. The vessels not only pass through the 

 nephric fold, but throw a solid process, the interior of which 

 consists of round or spindle-shaped cells. This is, according to 

 EÜCKERT, the equivalent of the pronephric glomerulus of Am- 

 phibia described by FÜrbringer. The development and decline 

 of these vessels go on parallel with those of the pronephric diver- 



1)1 will return to tills point again in future pages. 



2) Accoi'ding to Rabl's counting, tliis somite corresponds to Iiis forty-secnnd somite. 



