74 LEONARD W. WILLIAMS 



be surrounded by, the selerotomic tissue when the embryo has 

 about twenty segments, and in embryos of twenty-five segments 

 they are quite surrounded by selerotomic tissue. 



The notochordal processes of the sclerotomes now unite be- 

 neath the notochord, separating it and the neural tube from the 

 entoderm, and also unite beneath the aorta with the aortic proc- 

 esses, separating the aorta of each side from the entoderm. 



In embryos of forty segments (fig. 9) the dermomj^otome is 

 much larger than before and is now of irregular thickness, being 

 more than twice as thick at the junction of its middle and lower 

 thirds as in its upper fourth. This is due to the rapid expansion 

 of the tissue of the cutis plate which accompanies its transforma- 

 tion from an epithelioid into a reticular or mesenchymal form. 

 This transformation can be readily followed in transverse or fron- 

 tal sections, for, beginning at a point somewhat lateral to the cen- 

 ter of the cutis plate, the area of disintegration rapidly spreads 

 in all directions until it reaches the lower, anterior, and posterior 

 edges of the cutis plate. The history of the lower edge of the cutis 

 plate is brief, for, as Fischel has shown, the dermomyotome of 

 each of the prebrachial somites does not develop along its lateral 

 or lower edge a zone of growth ; consequently this edge is quickly 

 transformed into mesenchyma. The zones of growth along the 

 anterior and posterior edges of the dermomyotome continue for 

 some time to produce streams of cells which augment the mass 

 of mesenchyma, between the ectoderm and the myotome. The 

 cells arising from the two centers of growth on the posterior 

 edge of one and on the anterior edge of the following myotome 

 form a dense mass of mesenchyma which in frontal sections is 

 very conspicuous. 



The upper part (approximately one-fourth) of the cutis plate 

 retains an epithelioid aspect as is well shown in figure 121 in 

 Minot's "Human Embryology." It is differentiated, however, 

 into a loose distal portion and a thin dense basal layer containing 

 both dividing and resting nuclei. The latter layer forms the outer 

 boundary of the cavity of the segment which is now reduced to 

 a narrow cleft in the upper edge of the dermomyotome. I have 

 not been able to follow in detail the further history of the upper 



