BY JAS. P. HILL AND C. J. MARTIN. 
59 
stellate cells which become somewhat more compact below the 
ectoderm and immediately external to the edges of the medullary 
plate. The outer margin of this more compact portion of the 
mesoderm corresponds to the outer limit of the forward prolonga- 
tion of the protovertebral zone of mesoderm, and is visible in 
the photo-micrograph as the dark contour bounding this area. 
Bej^ond this line the mesoderm is split into the somatic and 
splanchnic layers. The somatic layer is composed of a single 
layer of cells and is closely applied below the ectoderm; the 
splanchnic layer is thicker, especially where it is inbulged over 
the heart endothelium (figs. 5 and 6, spf.). The two layers unite 
again into a single layer a little way external to the lateral heart 
Anlage. The lateral extension of the unsplit mesoderm beyond 
the heart Anlage is, however, very small, so that the lateral 
extent of the coelom practically corresponds in this region with 
the lateral extent of the mesoderm. From this point backwards 
the mesoderm gradually extends more and more outwards until it 
reaches its maximum extension opposite the posterior end of the 
embryo. 
Behind the heart Anlagen proper the somatic layer of mesoderm 
becomes very much thicker than the splanchnic (figs. 7, 8, 10 and 
12), and it continues in this condition to the posterior end of the 
embryo. At the same time the two layers become more closely 
applied to each other and the coelom is reduced to a narrow cleft. 
J ust over the venous trunks leading to the heart Anlagen the 
two layers are unsplit, thus dividing the coelom into a more mesial 
and a more lateral portion. 
The splitting of the more mesially situated part of the lateral 
mesoderm becomes more indistinct posteriorly, so that opposite 
the anterior somites the mesial part of the ventral coelom is largely 
obliterated and there exists external to the somites a mass of 
unsplit mesoderm (fig. 8). 
The par-axial mesoderm immediately in front of the first pair 
of somites, though not transversely limited in front as a distinct 
segment, has essentially the same appearance in section as that of 
the first somite. It forms on each side a distinct and compact 
