62 LEONARD W. WILLIAMS 



Engert ('00) in the chick and Bardeen and Lewis ('01) in human 

 embryos, proved beyond reasonable doubt that Rahl and Pater- 

 son were in the right and that the ventral body musculature does 

 arise from the ventral part of the myotome or muscle lamella but 

 that the muscles of the limbs arise independently of the myotome. 

 Engert shows that all of the cutis plate except its extreme upper 

 and lower edges forms dermal mesenchyma and that these edges 

 become transformed en masse into myoblasts forming the much 

 thickened dorsal and ventral edges of the myotome. Bardeen 

 ('00) however, maintained, I believe wrongly, that the cutis plate 

 in t\]e pig gives rise only to myoblasts. 



My intention, when I began this paper, was to study the later 

 development of the notochord of the chick and the relation of the 

 notochord to the vertebrae, as a continuation of the work upon the 

 notochord published in 1907. I soon found, however, that more 

 knowledge of the structure and development of the somites was 

 necessary. I have therefore followed with great care the history 

 of one of the two somites of the second segment up to the time of its 

 transformation into the sclerotome, myotome, and dermatome. 

 The differences at the time of their origin between the second and 

 several other segments have been pointed out. A brief account of 

 the history of the tenth segment, and of the relation of its sclero- 

 tomes to the vertebrae is given, and finally the history of the 

 twenty-fifth and forty-fourth segments is described, in order to 

 emphasize the differences, most of which are well known, between 

 the occipital, cervical, trunk, and caudal segments. 



THE SECOND SOMITE 



Rabl ('89) in his extensive work upon the early history of the 

 mesoderm has shown that, shortly before the origin of the first 

 segment, the mesoderm of the chick is represented by a sheet of 

 syncytial tissue which extends in all directions from the primi- 

 tive streak. Centrally it is thick and contains numerous closely 

 packed but irregularly arranged nuclei. Peripherally it gradually 

 becomes thinner until, near the inner edge of the area vasculosa, 



