THE SOMITE8 OF THE CHICK 57 



or intervertebral condensations are really masses of loose tissue 

 which become the vertebral centra. Gegenbaiir also saw that in 

 birds and reptiles (as is also true in mammals) the tissue around 

 the notochord forms a membranous, ''skeletogenous" or "peri- 

 chordal" sheath which is replaced by a cartilaginous or precar- 

 tilaginous sheath, which Minot named the chondrostyle. Re- 

 segmentation is effected by the formation of joints in the chon- 

 drostyle. 



In 1868 His published a new account of the somites of the chick. 

 He found that each somite is primarily a flat quadrangular body 

 consisting of a core and cortex; the former is a small cluster of 

 irregular rounded cells; the latter is composed of fusiform radiat- 

 ing cells attached to one another only at base. Each cell bears 

 peripherally a free projecting process. The more posterior 

 somites differ in several respects from the anterior or first formed 

 somites. They are cubical, or nearly so, their walls have more 

 epithelial characters, and their cores are larger. His like Remak 

 believed that in the chick the dorsal lamella is entirely con- 

 verted into voluntary muscle, and Bardeen maintains that this 

 is also true in the pig; nevertheless, this view has not been gener- 

 ally accepted. The credit of showing that the spinal ganglia arise 

 from the neural crest, not from the sclerotome, belongs to His but 

 he failed to see that the sympathetic ganglia arise in the same 

 manner. He believed wrongly that the core of the somite forms 

 the sympathetic ganglion and that the ventral wall of the somite 

 forms only the muscular coat of the aorta. All connective tis- 

 sues, according to his now abandoned parablast theory, arise from 

 the extraembryonic mesoderm and migrate along the blood 

 vessels into the spaces between the entoderm, ectoderm, neural 

 tube, notochord, and somites. Consequently His believed that 

 Remak's theory of resegmentation of the vertebral column was 

 wholly without foundation. Goette, however, in 1875 pointed 

 out that the tissues of the vertebral column really do arise from 

 the sclerotomes. 



Froriep ('83) found in each segment a lyre-shaped mass of 

 dense mesenchyma, which Bardeen has recently named the 

 scleromere. This extends from a midsegmental point below the 



