78 
JxVNE I. ROBERTSON. 
Jl 11 d i m e 11 1 s of the Vitelline Veins, Heart and 
Aorta. — Simultaneously, with the progressive delamination 
of the anterior margins of the lateral mesoderm from the 
surface of the yolk, two slender irregular strands of cells 
(Text-fig. 4, V. c.) appear, one on either side, between the 
yolk and the splanchnic layer of mesoderm. These are em- 
Text-fig. 5. 
Diagram illustrating the relations of the heart-rudiment to the 
mesoderm layers and coelom, co. Coelom, ec. Ectoderm. 
en. Endoderm yolk. li. Heart, my. Myocardium. ]ier. c. 
Pericardiac coelom, som. m. Somatic mesoderm, sp?. m. Splanch- 
nic mesoderm. V. C. Vessel cells, v.mes. Ventral mesocardium. 
bryonic endothelial cells, and they extend inwards across 
the anterior surface of the yolk simultaneously with the 
development of the mesoderm and ccelom. These cells are 
cotnparatively large and heavily yolked, and except peri- 
pherally, where they are continuous with the inner meso- 
derm layers over the lateral surfaces of the yolk, they are 
quite distinct from the endoderm and mesoderm, between 
which they lie. 
