HEAD-FOLD TO TWELVE SOMITES 12 
In the region of the area pellucida the entoderm is very thin; at 
its boundary it passes rather abruptly into the large rounded vesi- 
cular cells of the yolk-sac entoderm, which becomes continuous 
at the margin of the vascular area with the germ-wall; the 
latter continues to the periphery where it merges in the undifferen- 
tiated cell-mass (zone of junction) (Figs. 68 A-68 E). The large 
neural tube is not yet closed. Beneath the neural tube is a sec- 
tion of the solid rod-like notochord. 
Fic. 67. — Median longitudinal section of the head of an embryo of 13 s. 
Ectam., Ectamnion. F. B., Fore-brain. H. B., Hind-brain. Inf., In- 
fundibulum. M. B., Mid-brain.  pr’e. pl., Precardial plate. T. p., Tuber- 
culum posterius. Other abbreviations as before. 
The mesoderm (Fig. 68 A, B, C) lies between the parts already 
named; it consists on each side of the middle line of the following 
parts: (1) the mesoblastic somite, a block of cells that radiate 
from a central cavity filled with irregularly disposed cells; (2) the 
intermediate cell-mass or nephrotome, forming a narrow connect- 
ing bridge between the somite and the lateral plate; (3) the 
lateral plate, split into two layers, external, known as the somatic 
layer, and internal or splanchnic layer. The cavity between the 
two layers is the celome or body-cavity; it is very narrow next the 
nephrotome, but widens as it extends laterally to the margin 
of the vascular area, and is divided by various strands of cells 
extending from somatic to splanchnic layers, thus indicating its 
origin by fusion of ccelomic vesicles. 
The ectoderm plus the somatic layer constitute the somato- 
pleure, from which the body-wall, amnion, and chorion are derived, 
and the entoderm plus the splanchnic layer form the splanchno- 
