246 T. H. Burtenp, 
With regard to the first pronephric segment: 
for Torpedo Rickert gives the sixth or seventh, 
Pristiurus VAN WIJHE sixth ?, 
Rash seventh, 
Acanthias LAGUESSE seventh. 
In this respect Scyllium agrees with Pristiurus and Acanthias. 
Nature of the pronephric rudiment. 
Undoubtedly the conclusions of RÜCKERT, van WIJHE and 
C. RaBz have played an important rôle in the formulation of 
theoretical interpretations of kidney development, and upon the 
relationship between the pro-, meso- and meta-nephros. It seems 
to me rather unfortunate that their conclusions, by no means un- 
animous, have not been confirmed or reconciled. 
In the first place, the exact origin of the rudiment is by no 
means clear. The term “Urwirbel” which these writers used had 
not quite the meaning which it has today. Thus BALFOUR, in his 
account of Elasmobranch development writes as follows: “The cavity 
in each of the protovertebrae is formed of a narrowed dorsal and 
a dilated ventral segment, the latter on the level of the dorsal 
aorta”. The protovertebra is the structure which German authors 
call the “Urwirbel”; the dilated ventral cavity is what has been 
described as the Segmentalbläschen by van Wie, and is the cavity 
of the nephrotome, a structure which Srpewick first discovered in 
the chick. Rast, like Batrour, seems to include the Urwirbel- 
kommunikation in the term Urwirbel. Thus — “Als Urwirbel- 
kommunikationen habe ich im Vorhergehenden die ventralen Theile 
der Urwirbel bezeichnet, welche zur Verbindung mit den Seiten- 
platten dienen. Rückerr hat dafür den Namen Nephrotome ein- 
geführt; da aber, wie wir sehen werden, nur ein ganz bestimmter 
Abschnitt der Urwirbelkommunikationen zu den Urnierenkanälchen 
wird, und da ferner auch nicht alle Urwirbelkommunikationen diese 
Umbildung erfahren, so erscheint mir die von RÜCKERT eingeführte 
Bezeichnung nicht zutreffend”. — What RaBz meant by “Urwirbel”, 
Ferix speaks of as “primäre Ursegment”, and this later becomes 
differentiated into a “secundäres Ursegment” and a Nephrotome. 
Whatever interpretation. we put upon the use of the term 
“Urwirbel” as used by Rast, his statement that the pronephros 
arises from it is incorrect, since the Urwirbel is always metameric 
