244 T. H. Burtenp, 
Comparison of the main results with those of RÜCKERT, 
VAN WIJHE, C. RABL, and LAGUESSE. 
First appearance ofthe pronephric rudiment. 
In Scyllium canicula the first traces of a pronephros are to be 
found in embryos with 24 or 25 protovertebrae, in which the first 
gill-pouch is present, but not yet open. This agrees closely with 
the work of C. Rast on Pristiurus, in which the first trace of a 
pronephros is found in embryos with from 25 to 27 protovertebrae: 
van Wine gives 27 protovertebrae as the stage of the first ap- 
pearance in Pristiurus. This corresponds to stage H of Bazrour for 
Pristiurus, in which the first gill-cleft is nearly open (RÜCKERT, 
VAN WIJHE). 
In Torpedo the pronephros first appears at stage J of BALFOUR 
(RÜCKERT). | 
In Acanthias embryos LAGUESSE saw the first indications of the 
rudiment in embryos with about 37 protovertebrae, whereas GREGORY 
saw them in an embryo with 25 somites. 
Position of the pronephric rudiment. 
On this point there is a certain amount of difference in the 
accounts of various authors, even when dealing with the same 
species. This variation is probably due to two causes: 
1. The slight individual variations which may occur in a species, 
particularly if the embryos are slightly abnormal, as is of frequent 
occurrence when artificially reared. 
2. The different interpretations possible of the precise limits of 
the pronephros. 
Considering the mesodermal outgrowth: in Seyllium canicula it 
occurs on each side in the region opposite the seventh to the tenth 
metaotic protovertebra. Rasu’s account for Pristiurus agrees with 
my own for Scyllium: RaBz also states that the pronephros may 
exceptionally be limited to three segments — “nur in seltenen Aus- 
nahmefällen beträgt die Zahl der Vornierensegmente bloß drei”. 
Van Wie says that the fourth apparent protovertebra in the 
early embryo is really the first trunk-segment, and he concludes 
that in a Pristiurus embryo with 27 somites the pronephros is 
situated in the hinder part of the third, the whole of the fourth 
