3I0IIPH0L0GY OF CYCLOSTOMATA. 377 



The authors who have investigated the development of 

 Petromyzon embryos step by step, are W. Scott, Goette, Shipley, 

 and V. KuPFFEE. Their opinions are, however, somewhat 

 divergent. Scott ('82) derives the pronephric tubules from the 

 segmental duct which is, according to liim, brought about by 

 the difiereutiation in situ of the cells forming the proxhnal 

 margin of the lateral plate. The process takes place in the 

 whole extent at the same time. At certain points (segmental ?) 

 of the duct thus formed, evaginations are produced out of it ; 

 these evaginations subsequently open into the body-cavity and 

 establish the nephrostomes which are, according to Scott, 

 found from two to three pairs in number. At about the stage 

 in which the funnels are formed, he observed a pair of 

 glomeruli. 



" In most respects," Shipley's observations ('87) " confirm 

 his " (Scott's). But " on the origin of the ciliated funnels, the 

 results differ from Scott's " and agree witlî those of Furbringer 

 (Amphibian pronephros ?). According to Shipley, '* in the region 

 of the heart, where the body-cavity has already appeared, its 

 origin {i.e., of the segmental duct) seems to ])e somewhat different. 

 The lumen of the segmental duct here becomes continuous with a 

 groove in the parietal peritoneum, lying near the angle where 

 the somatopleure and the splanchnopleure diverge. When this 

 groove closes it leaves four or five openings which persist as the 

 openings of the ciliated funnels" (p. 20). 



v. KuPFFER^^ ('88) observed, in P. planeri, the three pairs of 

 the tubules arising from three distinct evaginations of the parietal 



1)1 know this paper uuly by Llie abstruct iii : JahresbericliL ii. die. Fortschr. d. Anal, 

 u. riiysiul., Ed. 17. ISbO. 



