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JANE I. ROBERTSON. 
sliorteiied. Meanwhile, as the adult condition is attained, 
the long axis of the ventral aorta becomes horizontal. 
Carotid Arteries. — The carotid arteries appear early as 
prolongations forwards of the dorsal aortic roots passing 
ventral to the anterior cardinal veins (Text-fig. 23 a. Car.). 
As development proceeds they come to lie below the otocysts, 
and give off an external branch which is distributed with the 
developing trigeminal nerve (Text-fig. 23 c, Gar. e.), after 
which they pass sharply inwards to the ventro-lateral 
surfaces of the brain. An anastomosis occurs between the 
two internal carotid vessels below the hind brain, and then, 
after passing forwards a little, they anastomose on each side 
with the vertebro-cerebral artery (see below) round the 
hypophysis, and are then distributed to the mid- and fore- 
brains. 
V e r t e b r o-c erebral Arteries . — About Stage 28 two little 
vessels appear on the dorsal surface of the dorsal aortic roots 
passing to the ventral surface of the posterior part of the hind 
brain. These vessels, the vertebro-cerebral arteries, are pro- 
longed along the ventral surface of the hind-brain and 
anastomose at intervals along their course (Text-fig. 2, V.G.A). 
Finally they reach well on to the dorsum of the hypophysis, 
forming a single median trunk, and then, separating again 
practically at right angles, curve round on either side and 
join the internal carotid vessels on the sides of the mid-brain. 
A little later each develops at its point of entry to the hind- 
brain a posterior or vertebral branch, that is continued back- 
wards along the ventral surface of the spinal cord as far back 
as the origins of the first pair of segmental arteries posterior 
to the subclavian vessels. Posterior to this, each pair of 
segmental arteries supplies a twig to these vertebral vessels. 
Homologous vessels are described for Lacerta and Tropi- 
donotus (1). 
Coeliac Artery. — The arteries to the pronephric glomeruli 
appear on the formation of the dorsal aorta (Stages 24-25). 
They enter the glomeruli dorsally, and after winding through- 
out their length, leave them veutrally and disappear among 
