The pronephros of Scyllium canicula. 245 
and the front part of the fifth trunk-somites. Thus “Die drei vordersten, 
wohlbegrenzten Somite gehören also dem Kopfe an und erst das 
vierte ist das vorderste Rumpfsomit.... Der Pronephros... erscheint 
nämlich als eine Ausstülpung der Somatopleura im blasenförmig auf- 
getriebenen unteren Teile des dritten, vierten und fünften Rumpf- 
somites.” Hence we are led to the conclusion that the rudiment in 
Pristiurus is situated in the sixth, seventh, and eighth metaotic seg- 
ments, although van Wuune’s figures clearly show it to be in the 
seventh, eighth, and ninth metaotic segments. RaBz noticed this 
discrepancy in van Wunes account also. Moreover the last-menti- 
oned author states that in a Scyllium canicula embryo with 60 somites 
the pronephros has shrunk into the fifth trunk-segment, viz. the 
eighth metaotic. In my embryo I (with 67 or 68 protovertebrae) I 
still find the pronephros in segments VII and VIII. 
LaGuEsse found the pronephros in segments VII, VIII, IX, 
and X in embryos of Acanthias. 
The pronephros extends over: 
in Pristiurus 5 segments according to RÜCKERT, 
4 segments RABL, 
3 segments VAN WIJHE, 
in Seyllium catulus 5 segments _ RÜCKERT, 
3 segments VAN WIJHE, 
in Acanthias 4 segments LAGUESSE, 
6 segments (GREGORY, 
in Torpedo 7 segments RÜCKERT, 
in Raja clavata 3—5 segments VAN WIJHE, 
in Raja alba 8 segments RABL. 
Van WiHE also noticed that in these early embryos, at least in 
Pristiurus, the nephrocoel is a swollen cavity in the first 13 metaotic 
‘somites, in stage H of Bazrour. In embryos with 31 somites they 
are still further extended, but have quite disappeared in embryos 
with 35 somites: I find these Segmentalbläschen present at a much 
later stage in Scyllium. They appear to correspond to the cavities 
which, in the mesonephric region become the malpighian capsules, 
and they most probably represent transitory pronephric rudiments 
in the anterior body-segments. Van WIJHE says that these struc- 
tures are transitory and take no part in forming the Selachian 
excretory system. RaBz does not appear to mention these Segmental- 
bläschen. 
16* 
