The pronephros of Scyllium canicula. 227 
at a later period it is connected with the peritoneal epithelium or 
at least in close contact with it. The caudal end of the pronephric 
duct (according to this author), is connected with the ectoderm and 
the cloaca at the same time. In the degeneration of the pronephros 
only one tubule persists, the nephrostome of which results from the 
fusion of the original nephrostomes: this process is variable, but the 
atrophy must occur either at the anterior or posterior end of the 
pronephros. 
CARL Rast published, in 1896, the most complete and authentic 
account of the development of the pronephros in Selachii. He dealt 
mainly with Pristiurus embryos, but also examined embryos of Raja 
alba, etc. RaBz does not deny that the primitive excretory organ 
extended over the whole length of the coelom: he agrees, moreover, 
with Rickert, that pro- and mesonephric tubules arise from different 
portions of the mesoderm, and therefore cannot be homologous. This 
author insists upon four pronephric segments, not three, as the 
organ in Amphibia might lead one to expect. RABz makes moreover, 
what I believe to be a mistake in all the published accounts of the 
Selachian pronephros, — viz. he looks for metameric rudiments in 
a structure which is in reality groove-like, and, although these are 
not apparent as such, since the rudiments at least occur inter- 
segmentally as often as they occur segmentally, Rasu states that 
the metameric tubules may be identified by the arrangement of the 
nuclei in the “Vornierenwulst”, and by incipient evaginations (ne- 
phrostomes) of the nephrocoel into the “Vornierenwulst”. Rast’s 
work is chiefly valuable in that he identified the arteries (first 
discovered by PaunL Mayer in 1887) as the pronephric arteries, and 
gave a careful account of their number, position and fate in Pristi- 
urus embryos. He concludes that the second and third ultimately 
fuse to give rise to the arteria vitellina (arteria umbilicalis of 
P. Mayer), though the first and fourth may also participate in this 
process. The Sammelgang, according to RABL, appears as a string 
(sickle-shaped in section) which binds together the free ends of the 
tubules: it is obviously a growth of the lateral ends of the tubules, 
although he admits that its exact development is not quite clear. 
This string may not join all the pronephric rudiments, for it may 
happen that the first pronephric rudiment grows out and forms the 
duct. with which the other rudiments fuse at a somewhat later 
period. The pronephric duct is entirely mesodermal in origin and 
Zool. Jahrb. XXXVII. Abt. f. Anat. 15 
