On the Neural Crest of the Lamprey Embryo

Abstract

ABSTRACT Those parts of the branchial basket of the ammocoete larva that have chondrified by the time the animal is 7 mm. long are of neural crest origin. Those parts of the trabeculae cranii (anterior parachordals) of the ammocoete that have chondrified by this time are not of neural crest origin, nor do they depend upon the presence of the crest for their normal formation. The factors governing the development of the visceral arch skeleton are, as far as is known, equivalent to those operating in Amphibia. The pre-migratory crest of the branchial region is competent to form cartilage, but not determined to do so. It is probable that the normal development of the visceral arch skeleton is dependent upon the early establishment of contact between branchial ectoderm and endoderm. The cranial neural crest is necessary for the normal development of the ventral arteries of the head. In its absence they are much dilated. It is probable that the walls of these vessels are normally, at least in part, of crest origin. In the absence of the neural crest in the trunk region the dorsal fin of the larva is not formed. Absence of the neural crest leads to absence of the dorsal root ganglia in the trunk and, usually, to a reduction in size of those of the head. The neural crest is a source, if not the only source, of larval melanophores.