The pronephros of Scyllium canicula. 283 
Embryo D. 
41}, mm long. 26 or 27 protovertebrae visible. Fixed with corro- 
sive acetic and stained in Boraxcarmine. Mounted upon three 
slides having respectively 142, 132 and 113 sections. 
Left side Right side 
1,65—1,86 Ear 1,59—1,79 
1,95—1,106 I 1,94— 1,106 
1,106— 1,117 II 1,107—1,115 
1,117—1,128 III 1,116— 1,126 
1,128—1,139 IV 1,127—1,136 © 
1,139—2,6 V 1,137—2,4 
2,6—2,16 VI 2,5 —2,14 
2,16—2,24 VII 2,14—2,22 
2,24—2,32 VIII 2,22—2,30 
2,32—2,39 IX 2,30— 2,38 
2,39— 2,47 X 2,38—2,45 
3,6—3,9 XXVII 3,9—3,8 
Two gill-pouches are present but not yet open: a third pouch 
is somewhat less advanced than the first two. Neurenteric canal 
3,91— 3,102, Segmentalbläschen in segments I1?, IIIT—-XH, XIII? 
The first elear evidence of the left pronephrie rudiment is in 
section 2,19, and from this rudiment a duct separates off at 2,39 or 
2,40 which continues back to section 2,69 in close relation with the 
ectoderm, but not fused with it. The “Vornierendrüse” itself may 
be noticed as far back as 2,44 (see Plates 19 to 21 Figs. 19 to 41). 
Hence the pronephros extends over segments VII, VIII, IX and the 
anterior part of X on the left side, and the duct extends back to 
the middle of seg. XIV. 
On the right side the kidney rudiment begins at 2,16, the duct 
separates off at 2,33 and ends at 2,69: there are traces of the 
pronephros as far back as 2,43. Thus segments VII—X comprise 
the pronephros region on this side also at this stage of development. 
The duct extends back to the middle of seg. XIV. 
The blood-vessels are in process of formation in this embryo. 
An examination of Figs. 19—41 will show that the splanchnic meso- 
derm of the lateral plate region is further away from the gut wall 
on the right side than it is on the left side. Thus is anticipated 
the formation of a blood-vessel, the arteria vitellina (or arteria 
