THE EARLY DEVELOPMENT OF THE LATERAL LINE SYSTEM OF AMIA CALVA

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Abstract
1. In very young embryos there is present a thickening on the neural tube which consists of a mass of mes-ectoblast of neural crest origin. In this the auditory invagination lies embedded. This portion of the neural crest later becomes divided into two lobes between which the auditory organ lies. In a still later stage the two lobes have become thinned out into sheets of mesoblast which extend one into the hyoid gill arch and the other into the first branchial gill arch. The seventh and ninth cranial nerves are probably also formed from the neural crest. The intimate relation of the auditory invagination to these lobes of the neural crest produces a striking resemblance to the condition found in the sensory anlage in teleosts. Here a single elongated thickening is described as dividing by two transverse fissures to form the anlage of the branchial sense organ, the auditory organ and the lateral line system respectively. 2. The auditory and lateral line organs of Amia do not have a common anlage. The auditory organ arises before the lateral line and independently of it as an invagination of the nervous layer of the ectoderm and forms later a closed vesicle. 3. The lateral line appears first in an embryo in which the auditory organ is a closed vesicle. The four primary lines (supra-orbital, infra-orbital, opercular-mandibular, and post-auditory) all arise independently of one another and of the auditory organ as thickenings of the nervous layer of the ectoderm and unite later to form a continuous system.

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