The pronephros of Seyllium canicula. 249 
the stage referred to, the statements of van Wine and Rast only * 
imply that the pronephros is usually most prominent in its middle 
region. 
Rabu ascribes to the Sammelgang of Pristiurus a much greater 
independence, since it can (he says) grow out both cranialwards and 
caudalwards of itself. The Sammelgang need not join all the prone- 
phric rudiments, for it may happen that the first pronephric rudi- 
ment grows out as “primäre Harnleiter”, and the other rudiments 
join with it later. 
My observations on Scyllium canicula point to the belief that 
the pronephric tubules and duct arise simultaneously in the region 
of the seventh, eighth, ninth (and tenth?) metaotic protovertebrae, 
and that it is at a much later period — between the stage of 50 and 
60 protovertebrae — that a differentiation occurs into tubules 
proximally, and duct (Sammelgang) distally. 
The origin and growth of the pronephric duct and 
its junction with the cloaca. 
The portion of the pronephric duct which Feuıx calls the 
“mesodermale Endabschnitt” is said to arise in connection with, 
and by the backward growth of, the Sammelgang. RÜCKERT gives 
the beginning of this process in Torpedo at the four still-closed 
gill-pouch stage. In Pristiurus it begins, at the end of stage H 
(BALFOUR) viz. in embryos with 35 somites, at first only on one side 
(vax WIJHE, 1889), Rasy first notices its origin in Pristiurus embryos 
with 40 somites. As regards its growth opinions are quite opposed: 
VAN WIJHE, 1886 to 1898 the hind end of the Sammelgang 
Bearp, 1887 | LA El pe with the ectoderm: back- 
Happox, 1887 ne ward growth is effected by 
Rickert, 1888 splitting off of cells from the 
LAGUESsE, 1891 | ectoderm. 
Rickert |say that the primary “Harnleiter” is derived from the 
LAGUESSE | ectoderm only. 
VAN WIJHE derives it from ecto- and mesoderm. 
Thus he says — “Wer die Entstehung des Ganges aus dem 
Ektoderm annimmt, wie eine solche von den meisten neueren 
Forschern für verschiedene Wirbelthierklassen angegeben wird, der 
muss nach meiner Ansicht als erste Anlage des Ganges die Stelle 
