THE DEVELOPMENT OF SYMBUANCHUS MARMOKATUS. 31 
began. In its development and in its fully formed condition 
this pronephric chamber exactly resembles the Malpighian 
capsules of the mesonephros, though the former is larger 
than the latter. The glomerular rudiment is formed of deep- 
staining cells interspersed Avith blood-corpuscles, the whole 
forming a little clump lying in the cavity of the pro- 
nephric chamber, the lining of which is exceedingly thin. 
'Ihis clump lies on that side of the chamber which is nearest 
to the dorsal aorta rudiment. 
The cells of the archinephric duct rudiment are assuming 
an epithelial character, and in some places there is a lumen. 
Both ducts have become more deep-seated and now lie 
ventral to the dorsal aorta. The inter-renal vein appears at 
this stage. 
Stage 25. — The pronephros has increased greatly in size, 
and the two pronephric chambers with their respective 
glomeruli are very conspicuous structures in horizontal 
sections. They lie in contact with the paired aortic roots 
just as these are joining up to form the dorsal aorta. An 
opening from the short pronephric tubule into the archi- 
nephric duct is established, the duct being now perforate 
throughout its length. 
Stage 26. — The pronephros has reached its maximum 
size. The glomerulus of each chamber is conspicuous, and 
the opening into the tubule is funnel-shaped. There is no 
obvious boundary between tubule and duct. Anteriorly the 
latter is becoming coiled, posteriorly, it is quite straight. 
Its opening into the temporary cloaca has already been men- 
tioned in Part I. 
In Stage 26 the pronephros has become more deep-seated, 
lying immediately ventral to the dorsal aorta, which at this 
point gives off a coeliaco-mesenteric artery behind the aortic 
roots. The archinephric duct is surrounded by blood, the 
posterior cardinals anteriorly, and the inter-renal vein pos- 
teriorly, bathing the ducts on their inner side, while on the 
outer side is blood which communicates with the segmented 
somatic veins. There is no sign of the pseudo-lymphatic 
