66 LEONARD W. WILLIAMS 



I believe, however, that the cortex of all the somites of the chick 

 and, probably, of all birds and mammals, is an epithelioid syn- 

 cytium of this peculiar type. 



The upper wall of the somite is thin laterally where it is con- 

 tinuous with the upper layer of the lateral plate. Medially it 

 gradually becomes thicker, its medial edge being nearly twice 

 as thick as its lateral edge. The medial wall of the somite 

 immediately underlying the thick edge of the upper wall is quite 

 thin and is indented by a small groove-like evagination of the 

 cavity of the somite, (fig. 4, G.U.), for which I propose the name 

 upper myotomic groove. The appearance of this groove is the first 

 indication of the formation of the myotome. The cells at its 

 base apparently change from a cylindrical to an oval or spheri- 

 cal form in preparation for their subsequent longitudinal elon- 

 gation as they become definitely recognizable myoblasts. The 

 floor and the lower part of the medial and anterior walls have 

 fused with the core of the somite to form a mass of mesenchyma, 

 the sclerotome, or since it subsequently receives a very considera- 

 ble addition from other parts of the cortex, the primary sclero- 

 tome. The fusion between the core and cortex begins first at 

 the anterior end of the ventro-medial edge of the somite and, 

 owing partly to the expansion which accompanies the transforma- 

 tion of the tissue of the somite into mesenchyma, this angle grows 

 forward and medially more rapidly than the remainder of this 

 edge of the somite. 



In embryos of twelve segments, a second groove appears (fig. 5, 

 G.L.) in the medial wall of the second somite. This, however, 

 is an invagination of the wall and lies at a slightly lower level 

 than the upper myotomic groove. Since it marks the lower edge 

 of the muscle plate, I propose to call it the lower myotomic groove. 



The upper myotomic groove, I believe, has never been described 

 or figured. It is, however, neither as constant nor as conspicu- 

 ous as the lower groove which, although often figured, has not 

 been described or named. A cord of vascular cells partly fills 

 the lower myotomic groove. 



The myotome is represented by a narrow zone between the 

 two myotomic grooves which differs considerably from the 



