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The pronephros of Seyllium canicula. 255 
nephric region at this stage he would have found that the tubules 
are never segmentally arranged; that there is a solid connection 
between ostium 1 and 2, and between 2 and 3; in other words, the 
ostia are formed by the persistence of a cavity at intervals in a 
primitive pronephric groove, and not by the hollowing out of meta- 
meric pronephric rudiments. RaBz made figures of the successive 
conditions of the pronephros: his conclusions seem to be unsatisfactory 
because he has not rejected the condition in some of his embryos 
as abnormal. Thus his text-figure 9, as compared with the figs. 
which precede and succeed it, appears to represent an abnormal 
condition. Moreover I have been able to demonstrate that the 
tubules 2 and 3, if the latter be present, become closed off from 
the coelom, because traces of this closure persist for a short time 
as a solid connection with the peritoneum (cf. figs. 75 to 96). The 
fusion of the tubules begins, according to Ras, in a Pristiurus 
embryo with 62 somites, and is complete on the right side at the 
78 somite stage: on the left side it is not complete until the 
94 somite stage at the earliest, and it may be delayed until the 
embryo is 19 mm long. In Scyllium canicula the third nephrostome 
disappears on the right side between the stages of 68 and 70 proto- 
vertebrae: the second nephrostome on each side seems to atrophy 
before the embryo is 10 mm long. 
Theoretical conclusions reached from a study of the pronephros 
in the Dogfish as compared with the existing views on the 
pronephros in Vertebrates. 
The existing views upon the pronephros have been summarized 
in an earlier paper (The pronephros of Chrysemys marginata). I will 
therefore briefly summarize any conclusions of importance, which a 
study of the pronephros in the Dogfish seems to support. 
The pronephros is at first a continuous non-metameric structure 
originating as a groove in the somatic layer of the lateral plate 
mesoderm, not as metameric outgrowths in the protovertebra region. 
Both the portion which gives rise to tubules, and the portion 
which gives rise to the “Sammelgang” arise at the same time, and 
from the same rudiment. 
The whole of the pronephric duct is probably of the same nature 
as the “Sammelgang”, but has a modified development owing to 
its later appearance: it is entirely mesodermal in origin. 
