126 
JANE I. KOBERTSON. 
beyond the margins o£ the endodermic pharyngeal rudiment^ 
their walls are formed directly by the canalisation of the 
mesenchyme cells of the head region. The posterior cardinal 
veins and the glomerular arteries are formed from the 
mesenchyme layer already mentioned as lying between the 
Text-fig. 29. 
A. Section at the periphery of the yolk showing the mesoderm 
layers (characterised by smaller nuclei and finely fragmented 
yolk-grannles), ill-defined from the yolk, which has large scat- 
tered nuclei and large graiuiles. b. Section at periphery of 
the yolk showing the formation of the free blood-corpiiscles in 
the mesoderm layers. h. c. Primitive blood-corpuscles. ec. 
Ectoderm, m. Mesoderm. 
pronephros and the yolk^ while the dorsal aorta is formed by 
sclerotome cells that pass in from either side ventral to the 
hypocorda (Text-fig. 28 b, go.) In all these situations 
probably the young endothelial cells continue for a time to 
