LEPIDOSIREN PARADOXA. 
105 
When they first appear at either end of the heart, the non- 
niuscular septa are formed by proliferations of the local 
mesenchyme cells, but as they grow proximally away from 
their points of origin into the heart itself, the endothelial cells 
appear to take a larger and larger part in their formation. 
It is extremely difficult to dogmatise, however, as to the exact 
derivation of these septal cells, as the mesenchyme and endo- 
thelial elements much resemble one another, but undoubted 
proliferation of the endothelial cells does occur at the 
proximal ends of the growing septa. The endothelial origin 
of the endocardial cushions in the elasmobranchs would point 
to the probability of a similar origin in Lepi do siren. 
As far as any considerable changes in shape are concerned, 
the development of the heart may now be considered to be 
complete. The only change still taking place is that the 
ventricular and auricular walls continue for a time to become 
increasingly muscular. 
IV. Development op the Arteries. 
Aortic Arches. — The endothelial rudiments of the lateral 
ventral aortae are present about Stage 24 (24 segments) ; they 
pass from the mesial heart rudiment between the parietal 
layer of the pericardium and the lateral expansions of the 
pharyngeal rudiment sharply outwards doi sally, and a little 
backwards to the sides of the head. 
A little later. Stage 25, the rudiments of the four external 
gills appear on the dorso-lateral surfaces of the head, and the 
ventral aorta (Text-fig. 23 a, F. A.) on either side is pro- 
longed outwards and dorsally to their bases, giving an 
afferent branch to each of the four posterior gill-rudiments, 
there being no afferent aortic branches to the first and second 
(mandibular and hyoid) arches. Correlated, however, with 
the flattening of the embryo and the extreme lateral position 
of the gill-rudiments at the sides of the neck, the origins of 
the four posterior aortic arches are fused together, one behind 
the other at the outer extremity of the paired ventral aorta 
