34 
MONICA TAYLOR. 
it is very difficult to follow their history completely. These 
secondary tubules are not numerous in Sy mbranc bus. 
Stage 43. — The pronephric chambers with their glomeruli 
are still present, though insignificant structures, very much 
crushed out by the surrounding tissues. Their position is 
more posterior relative to the shoulder girdle. New meso- 
nephric tubule rudiments are still being formed. 
The urinary bladder reaches anteriorly almost to the com- 
mencement of the rectum, and posteriorly a blind sac-like 
dilatation protrudes into the tail. 
The Adult. — A dissection of the excretory system of an 
adult of twelve inches shows an elongated paired kidney lying 
on either side, and in close contact with the inter-renal vein. 
It extends from a point lying dorsal to the middle of the 
pericardium — to the rectum — and is flattened dorso-ventrally. 
At the beginning of the rectum (thirty-eighth trunk-seg- 
ment) it opens into the urinary bladder, which has the same 
diameter as the rest of the excretory organ. A small diver- 
ticulum of the bladder passes into the tail. The urinary 
opening is distinct from, but immediately posterior to, the 
anus. 
(c) Vasculae System. 
Rudiments of the dorsal aorta appear in Stage 19 as clear 
spaces lying between the nephrotomes and notochord. In 
Stage 21 the dorsal aorta, in which blood-corpuscles are be- 
ginning to appear, is still paired throughout its length. 
A sagittal section through Stage 22 shows that the blood 
from the yolk collects into a widened receptacle and then 
passes into a single and almost straight elongated vessel which 
expands somewhat just where it comes into connection with 
the first two visceral arches. The widened receptacle, the 
rudiment of the atrium, is quite anterior to the embryo. The 
rudiment of the ventral aorta communicates with that of the 
dorsal aorta by vessels which run up the mandibular and 
hyoid arches respectively. The blood reaches the yolk-sac 
