THE SOMITES OF THE CHICK 85 



the myotome. The corresponding ectodermal ridge, however, 

 has been elevated, especially its vertical portions, into an acumi- 

 nate septiim-like structure which in one case projects medially 

 from the bottom of the ectodermal invagination 50 microns, or 

 more than one-third of the distance from the bottom of the ecto- 

 dermal invagination to the myotome. 



The cutaneous blood vessels pass to the skin through the gaps 

 between the lower ends of the myotomes. 



The myotome is considerably thicker than before, and, except 

 at its upper and lower edges, is now triangular in frontal section. 

 Its outer surface is flat or slight^ convex and its medial surface 

 is now divided into two equal surfaces which, facing slightly for- 

 ward and backward, meet in a large obtuse angle that varies in 

 different levels from about 120° to 160°. The myoblasts of the 

 lateral part of the myotome are in a more advanced stage of devel- 

 opment than those of the medial portion, and they seem to stretch 

 from the antero-medial to the posterior medial surface of the 

 myotome. The myoblasts of the medial part of the myotome are 

 shorter and are more irregularly arranged than the more lateral 

 myoblasts. Those in immediate contact with the medial surfaces 

 near the medial angle of the myotome form an epithelioid layer. 



The transverse diameter or breadth of the intervertebral fissure 

 is considerably reduced owing to the encroachment of the medial 

 angle of the myotome. 



The axial mesenchyma is denser than in embryos of forty- 

 four segments, and there has now appeared an extensive midseg- 

 mental subnotochordal condensation which is continuous later- 

 ally with the dense anterior and posterior sclerotomic columns. 



The presence of the large spinal nerve and the sympathetic 

 ganglion in the anterior sclerotomic column interferes with the 

 original continuity of the anterior column and therefore Froriep 

 and Bardeen have conceived that the dense tissue of each seg- 

 ment forms a simple lyre-shaped mass extending from the median 

 midsegmental point laterally, upward, and backward. Froriep 

 calls this structure the primitive vertebral arch, and Bardeen 

 names it the scleromere. Remak names its central portion the 

 primitive vertebral centrum, and its lateral portions the verte- 



