76 LEONARD W. WILLIAMS 



vast majority of mitotic figures are longitudinal, but, as will be 

 seen in the figure, a few are transverse or are directed otherwise 

 than longitudinally. Bardeen ('00) maintained against the gen- 

 eral opinion that the mesenchyma between the myotome and the 

 ectoderm, in the pig at least, does not arise from the cutis plate 

 whose cells migrate in a mass and without losing their epithelial 

 arrangement, from the middle of the anterior border of the myo- 

 tome, backward, upward, and downward toward the correspond- 

 ing edges of the myotome where the cutis plate cells turn over the 

 edge of the muscle plate and, with the exception of a few which 

 degenerate, become myoblasts. The proof of this migration lies 

 in the gradual reduction in the size of the cutis plate; in the grad- 

 ual transition in structure between the muscle plate and the cutis 

 plate; and finally in the existence of an external membrane upon 

 the periphery of the cutis plate. Nevertheless, there is no doubt 

 that the cutis plate in the chick gives rise to the mass of mesen- 

 chyma in question, for not only can the gradual transformation 

 of the epithelioid cutis plate into mesenchyma be followed, but 

 there is also no path open by which this tissue can migrate from 

 the parietal plate. The ganglion of the vagus and the anterior 

 cardinal vein quite fill up the narrow slit between the lower edge 

 of the muscle plate and the ectoderm through which alone the 

 tissue could migrate. In the majority of the segments of the 

 chick, a flat wedge-shaped plate of mesenchyme projects upward 

 between the lower part of the cutis plate and the ectoderm, just 

 as is the case in mammals; but in these segments the structure 

 and history of the cutis plate is the same as in the second segment, 

 except that the cutis plate has many more cells, and there is every 

 reason to believe that it produces mesenchyma. I have not had 

 an opportunity to follow the development of the cutis plate in 

 any mammal with sufficient care to warrant a positive assertion, 

 nevertheless, the similarity in structure and development of the 

 cutis plate in birds and mammals leads me to doubt Bardeen's 

 conclusions. 



The sclerotome of this somite has fused completely with the 

 adjacent sclerotomes, but its anterior boundary is indicated by the 

 first intersegmental vein and its posterior boundary by the first 



