THE SOMITES OF THE CHICK 71 



ends of the somite, the enlargement of the third part of the ante- 

 rior cardinal vein and the resulting separation of the dorsal la- 

 mella and the parietal plate, the migration laterally of the neural 

 crest cells and the proliferation from the edges of the dorsal 

 lamella of cells which contribute to the growth of the myotome 

 and sclerotome. The lower myotomic groov^e, which is now 

 scarcely more than a cleft, and the myotome underlie the medial 

 half of the dorsal lamella and consequently the sclerotome is con- 

 nected with the dorsal lamella only by a stalk whose diameter is 

 somewhat less than half the length of the somite. 



The sclerotome of this segment now begins to fuse with the ad- 

 jacent sclerotomes, and in all later embryos it is impossible to 

 distinguish the boundaries of the sclerotomes of the first five or 

 six segments. It is important, therefore, to note the relation of 

 the sclerotome to surrounding structures. Dorsally it is bounded 

 by the dorsal lamella. Laterally it is attached to the somatic 

 or parietal and the splanchnic or visceral layers of the lateral 

 plate. Medially it is in contact with the neural tube and the 

 notochord. Its ventral surface is in contact with the aorta and 

 bears the notochordal and aortic processes which come into con- 

 tact with the entoderm on each side of the aorta. The dorsal 

 part of the anterior surface of the sclerotome is in contact with 

 the transverse portion of the anterior cardinal vein, its ventral 

 part slopes forward and downward so that the notochordal proc- 

 ess (Text-fig. 1.) projects forward beyond the plane of the upper 

 part of the intersegmental fissure, one third the length of the 

 somite. The posterior surface of the sclerotome inclines sharply 

 forward and downward so as to allow the notochordal process of 

 the following sclerotome to project forward under the second 

 somite. 



The capillary vessels between the sclerotomes and between them 

 and the neural tube have begun to sink into the sclerotomes, and 

 thus the sclerotomic tissue receives its first blood-vessels. 



In embryos of tweniy-five segments the dorsal lamella (fig. 8) 

 inclines downward laterally at an angle of about 45° with the 

 median plane. The myotome has extended laterally so as to 

 unite with the greater part of the turned under lateral edge of the 



