394 



THE FURTHER DIFFERENTIATION OF THE 



half of the spinal cord is removed, the dorsal nerve-roots are un- 

 affected, and their nerves possess sheaths in the normal way.^ 



Removal of the neural crest in the region of the head leads to re- 

 sults which are in many ways remarkable, and difficult to interpret. 

 It is found that embryos of Amhly stoma punctatum from which the 

 neural crest of the head has been extirpated on one side show de- 

 ficient chondrification of the anterior part of the trabecula cranii 

 and of the cartilages of the visceral arches, including the jaws and 



7.^*77 



Fig. 190 



Left side view of the chondrocranium of a larva of Amblystoma showing (shaded 

 by dots) the regions which fail to develop after extirpation of the neural crest. 

 Au.cap. auditory capsule; B.oc. basal plate; Cbr. ceratobranchial ; Chy. cerato- 

 hyal; C.Tr. orbital cartilage; Ex.oc.M. oculomotor nerve foramen; Ex.op.N. 

 optic nerve foramen; M. Meckel's cartilage; Q. quadrate; Tr.B. trabecula; 

 Vert, first vertebra; i Bb., 2 Bb. first, second basibranchial ; 1-4 Ebr. first to 

 fourth epibranchial. (From Mangold, Ergebn. der Biol, in, 1928, after Stone.) 



branchial arch skeleton. ^ These results have been confirmed on 

 Amblystoma mexicanum^ and Rana.^ It is known that derivatives 

 of the cells of the neural crest extend ventrally at early stages into 

 the region of the visceral arches, and it would seem from these ex- 

 periments that these cells became directly converted into cartilage 

 cells. Conclusive proof would be obtained if intra vitam stains in 

 the neural crest at the neurula stage could be found in cartilage cells 

 at subsequent stages: some authors, indeed, working with de- 

 scriptive methods only, have professed to see special histological 



^ Harrison, 1924 b. 

 ^ Raven, 1931 b. 



2 Stone, 1926. 

 ^ Stone, 1929. 



