186 



THE DEVELOPMENT OF THE CHICK 



(Fig. 107) the primitive relations of the parts are still preserved. 

 In the twenty-sixth somite (Fig. 108) it will be seen that the 

 cells of the core and of the ventral and median wall of the somite 

 extending from the nephrotome to about the center of the neural 

 tube are becoming mesenchymal; they spread out towards the 

 notochord and the space between the latter and the dorsal aorta. 

 These cells constitute the sclerotome. The muscle plate extends 

 from the dorsal edge of the sclerotome to the dorso-median angle 

 of the wall of the somite, and the dermatome from this point 

 to the nephrotome. 



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Fig. 108. — Transverse section through the twenty-sixth somite of a 29 s 



embryo. (Same embryo as Fig. 107.) 



Derm., Dermatome. My., Myotome. Scler., Sclerotome. V. c. p., Pos- 

 terior cardinal vein. Other abbreviations as before. 



Fig. 109 is a section through the twentieth somite of the same 

 embryo. The sclerotome is entirely mesenchymal, and its cells 

 are extending between the notochord and aorta, and along the 

 sides of the neural tube. The muscle-plate has now bent over 

 so that its inner surface is being applied against the dermatome, 

 but there is still a considerable cavity (myocoele) between the 

 two, at the lateral angle of the dermo-myotomic plate. The 

 lateral edge of the dermatome is freed from the nephrotome, and 

 turns in to a slight extent. Other details are readily understood 

 from the figure. 



The growth of the free edge of the muscle-plate towards the 

 free lateral edge of the dermatome continues as illustrated in 



