84 LEONARD W. WILLIAMS 



or the posterior vertebra of the segment. These cohuiins are, I 

 beHeve, nothing more than centers of growth which have been cut 

 off by the lower myotomic groove from the zones of growth of the 

 anterior and posterior edges of the dorsal lamella of the somite. 

 They certainly contain (as Von Ebner pointed out in 1892, and 

 as I have maintained elsewhere) the anlagen of a large number of 

 structures. The anterior, however, differs largely from the pos- 

 terior column, owing, I believe, to the intrusion of the spinal 

 ganglion and nerve into it and the consequent interference with its 

 development. 



The cutis plate of each somite of the tenth segment of an embryo 

 of forty-four segments (H. E. C, no. 98) has given rise to a large 

 mass of loose dermal mesenchyma and to a wall-like peripheral 

 zone of denser mesenchyma which extends from the zone of pro- 

 liferation at the anterior, posterior, and dorsal edges of the myo- 

 tome and from the remnant of the lower edges of the cutis plate 

 to the ectoderm. The septum of denser mesenchyma is attached 

 to the ectodermal thickening or ridge along the anterior, posterior, 

 and lateral (or ventral) boundaries of the somite, and seems to 

 draw the ectoderm toward the myotome, causing a deep invagina- 

 tion of the ectoderm. The existence of this invagination around 

 three sides of the somites accounts for the greater conspicuous- 

 ness of the somites of entire embryos of three or four days than 

 of those of younger embryos. 



The outer lamella of the rapidly growdng upper edge of the der- 

 momyotome is not affected by the disintegration of the cutis 

 plate, for as Engert, has shown, this edge of the. dermomyotome 

 becomes enlarged, appearing in transverse sections like a pot- 

 hook or a shepherd's crook, and is transformed bodily into myo- 

 blasts. 



The sclerotomic tissue, both of the perichordal sheath and of 

 the sclerotomic columns, is considerably denser than before. 

 Sympathetic ganglia are now present in the tenth segment. 



In embryos of forty-eight to fifty-two segments (H. E. C. nos. 

 478, 483, and 526) the dense peripheral portion of the dermal mes- 

 enchyma has become much less conspicuous except along the 

 dorsal and the upper part of the anterior and posterior edges of 



