THE SOMITES OF THE CHICK 65 



His states that the most anterior somites do not contain cav- 

 ities, but there are cavities in the first three or four somites and 

 the communications between them and the coelom in embryos of 

 from ten to twenty segments were discovered by Dexter ('91). 

 This cavity and communication appear in the second somite of 

 embryos of five or six segments. Bonnet shows that the first 

 four somites of sheep embryos have a similar structure and con- 

 nection with the coelom. 



In embryos of nine segments, the neural tube is closed in the 

 region of the second segment and consequently the shape of each 

 of the second somites (fig. 4) is somewhat altered. Its medial 

 surface is nearly vertical, the upper surface is larger and is nearly 

 horizontal, and the lower surface is marked by a groove for the 

 aorta, which has now appeared and is rapidly enlarging. The 

 cortex or wall of the somite, particularly the dorsal wall, has a 

 very remarkable structure which has been recognized, I believe, 

 only by Held^ w^hose figures show this structure very beautifully 

 and accurately. While having the appearance of a simple or 

 stratified columnar epithelium, the cortex is really an epithelioid 

 syncytium of unusual character. The oval or elliptical nuclei 

 are imbedded in columns of cytoplasm which proximally, i.e., 

 toward the core, are continued into a dense basal layer of cyto- 

 plasm and distally end in irregular sparingly branched processes. 

 Certain of these distal processes unite so as to form a faint exter- 

 nal boundary of the cortex, but others extend freely into the space 

 between the somite and the ectoderm and other adjacent struc- 

 tures. Nuclei preparing to divide withdraw to the basal layer 

 of cytoplasm, leaving conspicuous gaps in the outer part of the wall. 

 I have been unable to find a single mitotic figure elsewhere in 

 the somitic cortex than in close proximity to the basal layer, and 

 it is interesting to note that practically all of the axes of the mi- 

 totic figures are parallel to the surfaces of the cortex. The struc- 

 ture described is quite unmistakable in the first five or six somites 

 which have relatively few cells, but posteriorly it is somewhat 

 obscured by the much greater number of cells in the cortex. 



' Figures 91-96 of birds, 98-99 of the rabbit. 

 The American Journal of Anatomy, Vol. 11. No. 1. 



