The pronephros of Seyllium canicula. 251 
fusing with another germinal layer, and furthermore we have a 
reasonable explanation of the like origin of both Sammelgang and 
Endabschnitt portions of the duct, the latter having suffered an 
arrested development, while the opening of the duct posteriorly — 
in a region where all the three germinal layers were primitively 
continuous — is not so improbable as are the earlier interpretations 
of the growth and relations of the pronephric duct. 
The greatest development of the pronephros. 
The pronephros never becomes so prominent in Selachii as it 
does for example in Amphibians: in Torpedo the structure extends 
over 7 segments and is greatest and thickest at the stage when 
four gill-pouches are present, but still closed. Tubule 5 is the 
largest, each tubule in front of it being somewhat smaller than the 
one immediately behind; tubules 6 and 7 are rather smaller 
than 5 (Rickert). In Pristiurus the pronephros is largest at the 
40—45 somite stage (Rabu). The pronephros probably reaches its 
greatest development in Scyllium canicula at the 40—45 somite 
stage also. 
The filtratory apparatus. 
Rasy gives an excellent account of the pronephric blood-supply 
in Pristiurus: from the aorta transversely directed vessels pass off 
in the pronephric region inter-segmentally, and these open into the 
arteria vitellina (Rabi) or arteria umbilicalis (PAun Mayer). The 
number of these pronephric arteries varies in Selachii in the same 
way as do the tubules: 
thus Pristiurus has 4 tubules, 
4 arteries, 
Torpedo has 7 tubules, 
7 arteries, 
and in Seyllium canicula 3 tubules, 
a arteries. 
The arteries develop on the right side only: on the left side 
there are corresponding outpushings of the aorta which never mature, 
and soon disappear. On the right side also a degeneration of the 
arteries soon sets in, and at a later stage one artery — which be- 
comes the vitelline artery — is alone present. But whereas RABz 
has stated that the vitelline artery arises by the fusion of pro- 
