THE SOMITES OF THE CHICK 67 



adjacent portions of the cortex of the somite. The cells or cell- 

 like elements of the myotome are shorter and are more closely 

 packed than those of the rest of the cortex. Moreover, the cells 

 of the myotome radiate from the upper myotomic groove, up- 

 ward, dorso-medially, medially, and ventro-medially. The upper 

 wall of the somite is somewhat thicker and its cells are more 

 closely placed than before. Its medial portion is overlaid by a 

 plate of neural crest cells. 



The communication between the coelom and the cavity of 

 the somite is larger than in younger embryos and is approximately 

 circular in cross section, i.e., as seen in sagittal series. It has now 

 reached perhaps its greatest development. The stalk of the 

 segment is deeply constricted both on its upper and its lower 

 surface, in front and behind the communication. 



The sclerotome is slightly larger, owing partly to the transfor- 

 mation of more of the anterior and medial walls of the somite into 

 mesenchyma. The posterior wall and the posterior ventral edge 

 of the somite are still epithelioid and have not contributed to the 

 formation of the sclerotome. The amount of expansion of the 

 sclerotome is shown in the model (text fig. 1) and can be indicated 

 precisely by the example of the second somite of an embryo of 

 thirteen somites whose sclerotome in sagittal section is longer by 

 nearly one-fourth than the dorsal wall, and is one-tenth longer than 

 the distance between the centers of the adjacent intersegmental 

 clefts. The aorta divides the lower part of the sclerotome into 

 two keel-shaped processes, the aortic or lateral (fig. 5, P. A.), 

 which projects downward and medialty between the aorta and the 

 pharynx, and the notochordal or medial process (fig. 5, P.N'.), 

 which, projecting toward the notochord, separates the aorta from 

 the neural tube.. Fig. 119 (p. 204) in Minot's ''Human Embry- 

 ology" shows that there is a similar projection forward of the lower 

 part of the sclerotome of the rabbit. 



In emhryos of fifteen segments (fig. 6) the most conspicuous 

 alteration of the second somite as compared with that of embryos 

 of twelve segments, is the deepening of the lower myotomic groove 

 and its extension upon the anterior and, to a slight extent, upon 

 the posterior surface of the somite. The groove is deep and nar- 



