THE SOMITES OF THE CHICK 91 



tal condensation. The ribs in the same way arise in the dense 

 tissue of the posterior sclerotomic columns. There is, therefore, 

 much reason to believe that in the chick cartilage arises froni 

 relatively loose as well as from dense tissue and also that dense 

 tissue always becomes somewhat looser before cartilage forms in 

 it. I have maintained in a former paper that cartilage arises 

 in this manner in the pig. 



There is a possibility that the forward growth of the lower part 

 of the sclerotome is a typical condition in vertebrates and, con- 

 sequently that the vertebrae are truly segmental structures. 



Paterson showed that the dorsal and ventral edges of the dermo- 

 myotomes of the trunk grow rapidly upward and downward re- 

 spectively into the membrana reuniens superior and inferior. 

 They are ultimately converted, as Engert shows, en masse into 

 myoblasts and form the thick dorsal and ventral edges of the 

 myotome. 



Rabl was right in asserting that the cutis plate in the chick 

 gives rise to dermal connective tissue, and I question the cor- 

 rectness of Bardeen's theory that the cutis plate in the pig pro- 

 duces only myoblasts. 



Finally, I wish to call attention to the circumstance that the 

 anterior cardinal vein arises in part from a branch of the aorta 

 that is comparable to the first intersegmental artery. 



