80 
JANE r. ROBERTSON. 
over a very shorfc space, and the heart-rudiment is corre- 
spondingly short also. A trifle later (Stage 24 +), a strand 
of embryonic endothelial cells is found, continuous with and 
anterior to the heart-rudiment immediately ventral to the 
pharyngeal rudiment, and from it again two similar strands 
pass, one on either side, sharply outwards and a little back- 
wards and dorsally to the sides of the head between the 
parietal pericardium and the lateral expansions of the pharyn- 
geal rudiment : these prolongations are the rudiments of 
the lateral ventral aortae (Text-fig. 6b, v. a.). Intra-cellular 
Text-fig. 7. 
IJ-V 
Expanding vitelline vessel and splanchnic layer of mesoderm 
bulging forwards into the coelom, co. Coelome. s/»Z. m. Splanch- 
nic mesoderm, v. v. Yitelline vessel. 
spaces, due to metabolic processes accompanied by the 
secretion of fluid, soon form in these various embryonic cells 
in the order of their appearance, that is to say, first in the 
vitelline veins, then in the heart, and finally in the rudiments 
of the ventral aortae (Text-figs. 4, 6a and b, r. c. and v. a.). 
These spaces increase in size, adjacent cells coalesce, and finally 
definite endothelial vessel-tubes result (Text-fig. 7, v. v.). 
Simultaneously with the expansion of the endothelial 
tubes the splanchnic mesoderm bulges before them into the 
coelomic cavities and affords a covering to the developing 
vessels (Text-fig. 7, s^l m.). As the middle cells of the heart- 
rudiment vacuolate and expand a trifle later than those at its 
lateral margins (Text-fig. 6a, v.c.), it has for a while a 
somewhat dumb-bell-like appearance on horizontal section. 
