BY JAS. P. HILL AND C. J. MARTIX. 59 



stellate cells which become somewhat more compact below the 

 ectoderm and nnmediately 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. 

 Be^'-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, spL). 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 ctelom 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 ctelom is reduced to a narrow cleft. 



Just over the venous trunks leading to the heart Anlagen tlie 

 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 



