LEFIDOSIREN PARADOXA. 
129 
in the thicker ventral mesoderm layer. Projected on a plane 
surface, this mnltinuclear region of the lateral mesoderm 
would have somewhat the outline of Text-fiof. .31. 
The mesoderm layer over the yolk now gives rise by 
repeated nuclear division to a thin outer layer of splanchnic 
mesothelium, and internal to this, to free rounded cells — the 
primitive blood-corpuscles — contained in irregular lacunar 
spaces formed by the outer walls of the syncytial masses 
(Text-fig. 30b, be.) 
The formation of blood-cells is most active where the 
mesoderm layer is thickest, and is therefore most noticeable 
in the scattered areas over the yolk and along the cardio- 
anal lines already mentioned. In these regions cell-nests of 
varying- sizes are formed, packed with round blood-cells 
floating in fluid plasma (Text-fig. 30 b and c, he.) 
These cell-nests communicate, forming irregular channels, 
and finally, the vitelline meshwork is established, bringing the 
whole extent of the vascular mesoderm elements into relation 
with one another. Anteriorly the vitelline system communi- 
cates with the veins along the margins of the yolk and 
caudally with the aorta and caudal vein. 
These observations on the development of the elements of 
the blood-vascular system in Lepidosiren point entirely to 
their being of mesodermic origin (6). 
Bibliography. 
1. Bemmelen, J. F. van. — “ Kon. Akad. v. Wet. Amsterdam,” ‘Afd. 
Naturk.,’ 1885. 
2. Boas, J. E. V. — “ Ueber Herz imd Arterienbogen bei Ceratodiis 
und Protopterus,” ‘Morph. Jahrb.,’ Bd. vi, 1880. 
3. “Ueber den Conus arteriosus und die Arterienbogen des 
Amphibien,” ibid., Bd. vii, 1882. 
4. “ Ueber den Conus arteriosus bei Butirinus und bei andereii 
Knochenfischen,” ibid., Bd. vi, 1880. 
6. “ Ueber Herz uud Arterienbogen bei* Ceratodus und 
Protopterus,” ibid., Bd. vi, 1880. 
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