Development and growth of the caudal tissues in ammocoetes

Abstract

The posterior regions of the tails of 7- to 135-mm ammocoetes of Petromyzon marinus and Entosphenus lamottenii were studied histologically to investigate the manner of development and growth of the notochord, the cartilaginous skeletal elements, and the muscles, during the larval life of these lampreys. It is concluded that the cellular material of the so-called primitive streak contributes to the linear posterior growth of the notochord. The linear growth of the fin rays involves incorporation of additional cellular material from the connective tissue at the distal extremities throughout the larval life. New myomeres are differentiated from the cells residing in the general connective tissue at the dorsal and ventral ends of the posterior caudal myotomes for about a year after hatching of the larva. The vesicular posterior end with its lumen lined by cuboidal ependymal cells is a characteristic feature of the morphology of a normally growing spinal cord, and not only that of a regenerating one.