MUSEUM OF COMPAKATIVE ZOOLOGY. 273 



plate as a continuous cord of cells at a time wheu ouly the first trace 

 of the segmental vesicles has appeared. Before the (3 or 4) anterior 

 segmental vesicles have entirely lost their connection with the body 

 cavity, they communicate distally with the lumen of the segmental duct, 

 and may therefore be regarded as typical nephrostomal canals. This 

 condition is never encountered in the posterior vesicles, which develop 

 independently of the coelom in the solid "Wolffian blastema, or middle 

 plate. In consequence of this diii'erence in the mode of development 

 of the anterior and posterior portions, Mihalkovics is of opinion that the 

 first three or four segmental vesicles represent a rudimentary pronephros. 



According to Strahl ('86), the segmental vesicles are budded oft" from 

 the ventral portions of the protovertebree, and gain secondarily a con- 

 nection with the body cavity ; the duct does not appear until the vesicles 

 are evident. 



Ostroumoff ('88'', p. 81) confirms for Phrynocephalus the observations 

 of Mihalkovics regarding the anterior segmental vesicles, although he is 

 unable to ascertain the precise number that communicate with the body 

 cavity. He also interprets these anterior vesicles as a pronephros. The 

 duct, however, first appears in disjointed fragments lying between 

 successive vesicles. 



According to Hoff'mann ('89), there develops in Reptiles a pronephros 

 similar to that described by Riickert ('88) for Selachii. It appears as a 

 series of evagiuations of the somatopleure. These are formed in the re- 

 gion where the proto vertebrae pass over into the lateral plates. The organ 

 extends over a variable number of somites (6-7 in Lacerta and 5-6 in Tro- 

 pidonotus). As protovertebx'se separate from the lateral plate, the pro- 

 uephric evaginations remain in connection with the former, except in the 

 case of the first outgrowth (L. agilis, in L. muralis the first two), which 

 forms for a time a single pronephric ostium. The most posterior out- 

 growth extends backwards, and forms the fundament of the segmental 

 duct. The fate of the several evaginations is diff"erent. The most ante- 

 rior and possibly the next following outgrowth abort at an early stage ; 

 the remaining evaginations become detached from the protovertebrse and 

 fuse with one another, thus forming a tube closed in front, but continu- 

 ous posteriorly with the segmental duct. Hoff'mann identifies these 

 evaginations with the segmental vesicles of Mihalkovics and "Weldon, 

 but asserts that these authors mistook for a separate fundament of the 

 segmental duct a blind backward prolongation of the evagination belong- 

 ing to the immediately preceding somite. These backward processes are 

 described by Riickert for Selachii. OstroumofF's ('88'', pp. 78, 79) state- 



VOL. XXI. NO. 3. 18 



