BY JAS. P. HILL AND C. J. MARTIN. 
61 
The dark area in the photo-micrograph just external to the 
somites is the optical expression of this thick somatic layer of 
mesoderm. The outer limit of the dark area marks the place 
where the latter becomes reduced to a single layer. 
The ventral coelom is, in the region of the 1st somite and just 
anterior to it, coextensive with the thickened portion of the 
somatic mesoderm seen in surface view, while posteriorly it 
extends out beyond the point where the latter becomes thin. 
Further back still the coelom gradually becomes reduced in extent 
until in the region of the 9th to the 13th somites the mesoderm 
is no longer split (fig. 9, mes.). 
Opposite the 14th or 15th somites the mesoderm again becomes 
split, the coelom extending close up to the intermediate cell mass 
(fig. 10). 
Behind the somites the protovertebral zones of mesoderm are 
directly continuous with the lateral plates, while the splitting of 
the mesoderm does not occur until some distance out (fig. 12). 
The mesoderm continues some distance beyond the hinder end 
of the primitive streak, and here the coelom ic cavities gradually 
extend inwards towards the mesial line and fuse with each other, 
so that the coelom forms a continuous space. In this region the 
tail fold of the amnion will probably be developed. 
Primitive streak : In surface view the notochord is seen to 
become gradually thicker at its posterior end and to terminate 
finally in a distinct longitudinal thickening situated about the 
middle of the sinus rhomboidalis. The continuation forwards of 
this enlargement to join the notochord is the head process of the 
primitive streak, while behind it is the primitive streak itself, 
just visible in the photo-micrograph as a whitish line. 
Sections through the primitive streak show that mesodermal 
cells are being rapidly proliferated off from the ectoderm forming 
the floor of the primitive groove along its whole extent, and that 
the lateral wings of mesoderm are directly continuous with this 
axial streak of cells (figs. 12, 13, and 24). At the anterior end 
of the primitive groove ectoderm, mesoderm and entoderm are 
fused together in the axial line (figs. 23 and 24) and form the 
