THE SOMITES OF THE CHICK 77 



definitive intersegmental artery and the second vein. Occasion- 

 ally a small artery occurs on one or both sides in the place of the 

 normally aborted first intersegmental artery. The outer part 

 of the sclerotome is somewhat denser than its medial portion. 



The four occipital segments have only rudiments of spinal gang- 

 lia, but the ganglionic commissure of the vagus extends backward 

 through the somites of each side to the aborting ganglia of the 

 fifth and sixth segments. No ventral nerve root forms in the 

 first segment but, as Chiarugi has shown, the hypoglossal nerve 

 has three roots belonging respectively to the second, third, and 

 fourth segments. 



Frontal sections of the myotome show that its upper and lower 

 edges differ considerably in form. Its upper part, in an embryo 

 of forty-four segments (H. E. C. no. 98) is of nearly uniform thick- 

 ness and at each end narrows sharply to an acute edge which is 

 separated by a narrow hyaline zone from the next myotome. The 

 middle of the myotome is thicker than its upper part and is sym- 

 metrically convex. Its anterior, and to a less degree its posterior 

 edge are obliquely truncated so that an acute wedge of sclerotomic 

 tissue projects between the ends of the adjacent myotomes. 

 The inner surface of the lower part of the myotome is more con- 

 vex than its outer surface, and wedges of sclerotomic tissue, ap- 

 parently the rudiments of the myosepta, separate this part of the 

 myotome from the adjacent myotomes. 



Transverse sections through the middle of the last formed 

 somite of embryos of two, five, ten, fifteen, twenty-five, thirty, and 

 fortj^-f our segments are represented in figures 1 and 10 to 15. The 

 somites of the different regions of the body show certain differ- 

 ences which, though not large, have not been sufficiently empha- 

 sized, for it is assumed that all somites have the same structure 

 and history. Certain investigators upon this assumption have 

 tried to bridge over gaps in the development of a particular seg- 

 ment by taking the structure of a more anterior or more posterior 

 one as a later or an earlier stage of development. Lillie, for 

 example, says (p. 185): ''The manner of origin of these parts (of 

 the somite) can be studied fully in an embryo of twenty-five or 

 thirty somites, by comparing the most posterior somites in which 



