LEPIDOSIREN PARADOXA. 
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lifctle gill is drained by a tiny efferent vessel which joins the 
dorsal carotid artery just at the point of divergence of its 
internal and external branches (Text-fig. 23 c, Hy. 2). 
The presence primarily of six aortic arches, of which the 
two first are incomplete, having no connection with the ventral 
aortae, may be compared with the early condition described 
for Lepidosteus (19). The later development of the four 
posterior aortic arches, however, with the formation of the 
lingual artery and the disappearance of the first and second 
arches, closely resembles the condition in Urodeles, and the 
lingual and dorsal carotid vessels of Lepi do siren are homo- 
logous with the external and internal carotids respectively of 
the former ( 3 ). 
Dorsal and Ventral Aortae. — The dorsal aortic roots, 
formed by the efferent branchial vessels on either side, join 
below the notochord in the region of the pronephric glomeruli 
to form the dorsal aorta (Stage 25, forty-seven seg’ments) 
Immediately before doing so they give a branch to each 
glomerulus. The dorsal aorta is now present throughout the 
greater length of the embryo ; quite anteriorly it is a patent 
dilated vessel, but further back it is a flattened tube, while 
quite posteriorly it is merely a little rod of yolky cells that are 
beginning to vacuolate. For a short period (Stage 26) the 
aorta bifurcates posteriorly and communicates with the poste- 
rior terminations of the posterior cardinal veins, while a little 
later (Stage 29) it extends backwards and anastomoses round 
the post-anal gut with the caudal vein. (A similar stage is 
recorded in the development of Polyp ter us (15)). These 
connections are soon lost, however, and finally the caudal 
aorta lies in the hgemal canal immediately dorsal to the caudal 
vein, owing to the post-anal gut having disappeared. 
Anteriorly with the absorption of the yolk and the general 
development of the embryo, the junction of the dorsal 
aortic roots gradually occurs further forward nearer the 
heart, so that finally the unpaired dorsal aorta extends 
some little distance in front of the pronephric region, while 
the length of the dorsal aortic roots is proportionately 
