The pronephros of Scyllium canicula. 235 
Embryo F. 
51}, mm long. 41 protovertebrae. Mounted upon 3 slides having 
respectively 146, 147 and 151 sections. 
Left side Right side 
1,79—1,100 Ear 1,69—1,90 
— 1,126 Ti —1,117 
1,126—1,137 II 1,118— 1,133 
1,136 —2,4 III 1,130— 1,146 
2,2—2,14 IV 1,144—2,11 
2,13—2,26 V 2,10—2,24 
2,26—2,37 VI 2,23—2,35 
2,38—2,47 VII 2,35—2,46 
2,47— 2,56 Avent 2,46—2,56 
2,57—2,66 1X 2,56—2,66 
2,65 — 2,78 xX 2,66—2,78 
The tail of this embryo is cut obliquely: the neurenteric canal 
is present: there are 3 gill-pouches present but the last pair is not 
yet open to the exterior. From the condition and number of the 
pronephric arteries it is probable that the pronephros reaches its 
greatest development in this embryonic stage. The pronephros on 
the left side begins at 2,38, the duct becomes separated off at 2,66 
and ends at 2,126. Hence the pronephros extends over segs. VII, 
VIII and IX: the duct reaches back to the middle of segment XVI. 
The right pronephros begins at 2.35 (approx.); the duct is separated 
from the mesoderm at 2,61, and ends at 2,122. Thus the pronephros 
has become still further reduced on the right side as compared with 
the condition in embryo E, and it now extends over VII, VIII and 
the front part of segment IX. Segmentalbläschen are present in 
segs. II?, III to about segment XIV. In this embryo the vitelline 
artery is well developed, beginning in section 2,30 and cut in many 
sections more posterior. Its upper wall compresses the splanchnic 
mesoderm against the somatic mesoderm lateral to the pronephros in 
places, and so tends to render the relations of the rudiment to the 
coelom less obvious. The position of the kidney rudiment, relative 
to the nephrotome and lateral plate regions, is also seen to change 
as one passes from the front end of the pronephros to the region 
where the duct separates off (cf. section 2,38 Fig. 50 with section 
2,57 Fig. 69 right side). 
