228 T. H. Burtenp, 
grows backwards by its own growth, due to rapid karyokinesis in 
Sammelgang and duct. The left kidney usually lags behind the 
right in development. In Pristiurus the pronephric arteries of the 
left side never mature. RaBz arrives at the same conclusion as 
VAN WisHE with regard to the origin of the coelomic opening of 
the Müllerian duct. 
The observations of BEARD were confined to a single embryo: 
he restrieted his attention to the question as to whether the pro- 
nephric duct is ectodermal or mesodermal. 
LaGuEssE (1891) confirms many of the observations of RÜCKERT 
and van Wie, using embryos of Acanthias. He derived the pro- 
nephric duct from the ectoderm. 
GREGORY (1897) ascribes the growth of the duct to two causes: 
a) splitting from the ectoderm, 
b) independent growth. 
Material and Observations. 
The description of the pronephros and its blood-supply is con- 
fined to the species Scylliwm canicula, although I have not restricted 
my examination to this species, since there were embryos of Seyllium 
catulus also at my disposal. 
The embryos are described under the letters A, B, K and this 
lettering also applies to their respective ages. It is to be clearly 
understood that these letters do not indicate stages of the Dogfish 
in any seuse comparable to the stages designated by these letters 
in Bazrour’s monograph upon Selachii. It did not occur to me at 
the time when the embryos were to be sectioned, that the number 
of protovertebrae would provide the best evidence of their age, as 
I felt that the length in millimetres would be sufficient to indicate 
the stage of development. I had no difficulty, however in counting 
the number of protovertebrae in the embryos described below after 
they had been sectioned, since none of the sections were lost in 
the process of mounting. 
The embryos were fixed in corrosive sublimate (23°) or in 
sublimate acetic: before sectioning they were usually stained in 
bulk for about 40 hours in Boraxcarmine, rinsed for five minutes 
in acid alcohol, and then put for an hour into 70°, 90°%,, and ab- 
solute alcohol successively. After dehydration the embryos were 
