The pronephros of Scyllium canicula. 295 
ment in Seyllium canicula falls into line with the method of development 
recorded for other Vertebrates, e. g. Chrysemys, and he suggested that 
the chief results of this work should be published at an early date. 
Historical. 
A list of the most important contributions to the early develop- 
ment of the kidney in Selachii is given at the end of this paper. 
The investigations date back to the work of RÜCKERT and van WIJHE 
about the year 1888. It will be, perhaps, convenient at this stage 
to summarize the chief results. 
Rickert (1888, 1889, 1892) gave particulars of the time of 
appearance, position, extent, and nature of the pronephros in Pristi- 
urus, Torpedo, and Scyllium catulus. He concluded that the pro- 
and mesonephros are not homodynamous organs because: 
a) they arise in a different way, 
b) mesonephric tubules arise at a later period in the pronephric 
region, | 
c) the mesonephros appears at a much later period in develop- 
ment than does the pronephros. 
Rickert also advanced the view that the pronephros had a 
greater extent in primitive forms than it has in extant species; 
that what was formerly the posterior region of the organ is now 
represented by the pronephrie duct; that the pronephros in Torpedo 
and Pristiurus fuses with, and receives cells from, the ectoderm; that 
the segmental pronephric tubules fuse distally to form a collecting 
duct (the Sammelrohr); that from the place where pronephros fuses 
with the ectoderm a duct grows backwards towards the cloaca by 
splitting off from the ectoderm; that in Torpedo the left kidney lags 
behind the right in development; that at the level of the cloaca the 
duct separates from the ectoderm and grows into the cloaca. 
Rückerr’s speculations upon the homology between the excretory 
tubules in Invertebrates and Vertebrates seem to be reflected rather 
too prominently in his actual observations, since his description of 
the mode of origin of the Sammelrohr, and the statement that the 
ectoderm participates in the formation of the pronephros, have never 
been satisfactorily verified since. 
Van WisHE began his work upon this subject as early as 1886, 
and published papers in 1886, 1888, 1889, and 1898, of which the 
most important is that of 1889. He worked more particularly upon 
