The pronephros of Scyllium canicula. 253 
The figures show that the nephrostomes first become distinguish- 
able on the left side in embryo I; the disappearance of the second 
is probable from the condition in Fig. G. On the right side there 
are three nephrostomes visible in I; two only in J, the third having 
closed up; and only one in K, the second showing signs of recent 
closure, since there is a solid connection between the pronephric 
duct and the coelom in the region previously occupied by the second 
tubule. 
Hence it appears that in Scyllium canicula the coelomic opening 
of the oviduct is due to the persistence of the groove-like nephrostome, 
which is originally situated in the greater part of segment VII and 
which may also extend into the extreme front end of segment VIII. 
Nephrostome 2 (which atrophies later) is situated near the 
posterior end of segment VIII (left side), and about the middle of, 
VIII (right side); whilst nephrostome 3 (which atrophies early) is 
present on the right side only in the posterior region of segment VIII. 
The vascular system is also instructive: embryo F appears to 
present the most highly developed condition, with three aortic branches 
on each? side arranged approximately in the following position: 
arteries on right side 
1. in front of somite VII 
2. at the anterior end of somite VIII 
3. in the posterior region of VIII. 
Thus the arteries alternate with the nephrostomes when the 
latter appear. 
In embryo K the second right pronephric artery persists some- 
what longer than it does in embryos I and J, and it may be that this 
artery (between nephrostomes 1 and 2) persists sufficiently long to 
be instrumental in effecting the closure of the second nephrostome. 
At any rate, the first pronephric artery is alone important in the 
formation of the vitelline artery. 
Hence my conclusions resemble those of RÜCKERT and van WIJHE, 
and conflict with those of Rann, with regard to the degenerative 
changes in the Selachian pronephros. Rast writes “Diese Tatsache 
ist van Wine und Rickert ganz entgangen; beide leiten die Dotter- 
arterie von einer einzigen Vornierenarterie ab und lassen die anderen 
zugrunde gehen. Ich habe guten Grund zu der Annahme, dab für 
Torpedo ganz ähnliches gilt wie für Pristiurus”. An examination of 
RagL's text-figures leads me to doubt his reason for saying that the 
