THE EARLY DEVELOPMENT OF THE CHICK 265 



eluded in the hinder part of the head region of the embryo, and 

 although no other definite boundary of the head appears for 

 some time, its future limit may be marked by the position of 

 the fourth somites. 



This division of the vertebral plate into somites expresses the 

 primary segmentation or metamerism of the body, and is funda- 

 mental, the metameric arrangement of other organs being 

 secondary and dependent upon this. Sections through the 

 somites (Fig. 102) show that their superficial cells are arranged 



my 



cr 



so 



FIG. 102. Transverse section through the last somite of a chick of about 

 forty-eight hours (twenty-nine pairs of somites). After Lillie. a, Lateral dorsal 

 aorta; c, crelom; cr, neural crest; my, myotome; n, notochord; nc, nerve cord; 

 ne, nephrotome; so, somatic mesoderm; sp, splanchnic mesoderm; W, Wolffian 

 duct. 



in the form of an epithelium, while the cells of the central parts 

 are loosely and irregularly arranged. This central portion, 

 although only virtually a cavity, is termed the myocoel; it cor- 

 responds, theoretically, with the region of the enterocoel of 

 other forms. 



The more lateral portions of the mesoderm, the lateral plates, 

 remain unsegmented. They are connected with the somites by 

 an intermediate, transitional band, also unsegmented in the 

 chick, known as the nephrotome or intermediate cell mass. 

 The dorsal and ventral surfaces of the nephrotome are con- 

 tinuous with the corresponding surfaces of the somites, the 

 separation between the two bodies being indicated by a de- 

 pression resulting chiefly from the thickening of the somites. 



