LEPIDOSIREN PAIIADOXA. 
77 
splits are present, one on either side, between its layers 
('rext-fig. 4, CO.). 
These two folds ot mesoderm, with their contained chinks, 
soon extend across the surface of the yolk, and meeting in 
the middle line, ventral to the pharyngeal rudiment, fuse to 
form the pericardiac portion of the coelom (Stage 24, Text- 
fig. 5 A, B and c, spl. m. and som. m.). 
Owing to the spherical shape of the yolk and to the close 
Text-fig. 4.' 
ec. 
Transverse section, Stage 23. Coelom just aj)pearing, columnar 
splanchnic mesoderm defined, vessel cells present, c. Columnar 
layer of mesoderm, co. Coelom, ec. Ectoderm, v. c. Vessel 
cells, y. Yolk. 
approximation of the embryo round it, the pericardiac coelom 
is, from the first, flattened, while it is curved transversely 
round the anterior surface of the yolk, and also dorsally a 
little on either side of the pharyngeal rudiment. Thus this 
part of the coelom is somewhat crescent-shaped, with the 
pharyngeal eudoderm resting between its horns, and also 
a little concave posteriorly owing to the curvature of the 
yolk. The walls of the pericardiac cavity consist of a single 
layer of cells, of which those of the splanchnic wall are 
distinctly columnar in shape, giving rise later to the myo- 
cardium, while those of the somatic wall are comparatively 
flattened cells from which the parietal pericardium is derived. 
^ Each division of the scale in this and succeeding figures represents 
•01 mm. 
