The pronephros of Scyllium canicula. 231 
1,106—1,116 Il 1,106—1,117 
1,113—1,123 III 1,117—1,126 
1193-1191 IV 1,126 —1,135 
1111140 V aad 
1,140—1,147 VI 1,144—1,151 
1,148—1,155 VII 1,151—1,160 
1,155—2,2 VIII 1,160—2,5 
29-910 IX 2,5—2,13 
2 10—2,18 x 213—2,21 
2,96 —2,99 XXV 2 99—9 102 
The first gill-pouch is just appearing at this stage. The neur- 
enteric canal extends from 2,130—2,140. 
In the above table the sections which pass through any particular 
protovertebrae on each side of the body are defined. The difficulty 
in drawing up this table arises from the fact that there is some 
uncertainty in ascertaining the first body-segment as distinct from 
the last head-segment. Assuming that there is only one proto- 
vertebra in front of section 1,105, as the sections of this series 
seem to show, the pronephros begins in the region of segment VII 
on each side and extends back into segment IX or X. This ob- 
servation agrees with that of Ragz for Pristiurus. Whichever seg- 
ment of the body this may be, I think that I have been able to 
identify it in the succeeding stages and I have marked it as seg- 
ment VII throughout. 
Segmentalbläschen in segments IT?, III—XII or XIII. 
The kidney rudiment at this stage takes the form of a con- 
tinuous unsegmented groove or solid outgrowth of the somatic meso- 
derm of the lateral plate region. The Figs. 2—18 (Plates 18—19) 
show that the groove-like nature of the rudiment is most obvious 
in the region opposite the middle of a protovertebra, and the fact 
that the rudiment is situated in the lateral plate region is most 
obvious in sections passing between successive protovertebrae. The 
rudiment extends on the left side from about section 1,150 to 2,8 
approximately: on the right side from about 1,151 to 2,21. Owing 
to the gradual rise to distinctness anteriorly and to the gradual 
loss of distinctness as we pass caudalwards, it is not easy to say 
with accuracy where the pronephros begins and ends at this stage. 
Perhaps this fact accounts for the slight discrepancies which are 
apparent upon an examination of the accounts of van WIJHE and 
RABL. 
