50 EARLY EMBRYOLOGY OF THE CHICK 



of transverse sections. Figure 17, B, shows the great dilation 

 of the coelom on either side of the anterior intestinal portal as 

 compared with its condition farther, caudad (Fig. 17, C). 

 Where the splanchnic mesoderm lies closely applied to the 

 entoderm at the lateral margins of the portal it is noticeably 

 thickened. It is from these areas of thickened splanchnic 

 mesoderm that the paired primordia of the heart will later 

 arise. 



In entire embryos of this age the thickened splanchnic 

 mesoderm can be made out as a dark band lying close against 

 the crescentic entodermal border of the anterior intestinal 

 portal (Fig. 16). If the preparation is favorably stained the 

 boundaries of the pericardial regions of the ccelom can be traced 

 (see Fig. 16). Following mesiad from the easily located thick- 

 ened areas, the mesodermic borders can be seen to extend from 

 either side parallel to the entodermic margins of the portal 

 nearly to the mid-line. They then turn cephalad. When they 

 encounter the ectodermal fold which constitutes the posterior 

 boundary of the subcephalic pocket they swing laterad parallel 

 with it and can be traced outside the embryonic region where 

 they constitute the cephalic borders of the anterior horns of 

 the mesoderm (see also Fig. 27, A). 



The portion of the coelom, the borders of which we have just 

 located between the subcephalic pocket and the anterior in- 

 testinal portal, is an important landmark from another stand- 

 point than the part it is destined to play in the formation of the 

 pericardial region. It is the most cephalic part of the ccelom. 

 There is no ccelom in the head. In the head region the meso- 

 derm is not aggregated into definite masses or coherent cell 

 layers. The mesodermic structures of the head are derived 

 from cells which migrate into the cephalic region from the meso- 

 derm lying farther caudally. These migrating cells are termed 

 mesenchymal cells in distinction to the more definitely aggre- 

 gated cell layers of the mesoderm. By careful focusing on the 

 whole-mount the mesenchyme of the head can be seen as an 

 indefinite mass lying between the superficial ectoderm and the 

 entoderm of the fore-gut. The distribution of the mesenchymal 

 cells and the characteristic irregularity of shape correlated 

 with their active amoeboid movement may be readily made out 

 from sections (Fig. 17, A). 



