THE SOMITES OF THE CHICK 73 



As Rabl and others have shown, the cells of this area begin to 

 separate slightly and to send out free distal protoplasmic processes 

 toward the ectoderm. At the same time, I find that a few cells 

 not in mitosis withdraw to the basal layer of cytoplasm which 

 as before is separated from the muscle plate by a cleft, the remains 

 of the cavity of the somite. 



The third portion of the anterior cardinal vein (fig. 8, C.A.) 

 is still farther from the ectoderm and is now separated from the 

 aorta only by a rather thin sheet of mesenchyma. It extends 

 outward only to the level of the upper surface of the muscle plate, 

 and it is separated from the ectoderm by the lower edge of the 

 dermal plate and, ventrally, by a part of the mass of neural crest 

 cells which is forming the ganglion of the vagus. The lateral, 

 anterior, and posterior boundaries of the somite and later of the 

 cutis plate, are indicated by a small acute ridge (fig. 8, R.) upon 

 the inner surface of the ectoderm. This ridge now marks the 

 boundary between the cutis plate (fig. 8, D) and the neural crest 

 cells (N.C.) A few isolated neural crest cells can be seen mi- 

 grating laterallj' from the roof of the neural tube. 



Evans states that "The center of each sclerotome is, on its upper 

 surface, supplied by a sheet of closely anastomosed capillaries; 

 but the outer divisions of the sclerotome are not so supplied. 

 There capillaries are absent for a considerable time, so that the 

 vertebral column presents a succession of vascular and non-vas- 

 cular zones, the former areas in each case overlying the segmental 

 vessels" ('09, 2, p. 515). This statement does not hold good 

 for the sclerotomes of the four cephalic segments owing doubtless 

 to the fact that the cephalic sclerotomes differ considerably in 

 structure (compare p. 77) from the spinal sclerotomes. The scle- 

 rotomic mesenchyma of the head is divided into a vascular outer 

 and upper zone above the aorta and lateral to the intersegmental 

 arteries and a non- vascular zone lying beneath the neural tube and 

 the notochord. Evans' figure (cf. '09, 1, ng. 3) shows that the 

 richest capillary plexus lies in a longitudinal zone at the side of 

 the neural tube and between it and the medial or dorso-medial 

 surface of the sclerotomes. The capillaries of this plexus, like 

 other capillaries around the sclerotomes, begin to sink into, or to 



