LATERAL LINE SYSTEM OF AMIA CALVA. 2J 



In an embryo that has just hatched (Fig. 1 1) the supra-orbital 

 line extends cranially to a point just dorsal to the eye and is a 

 continuous cord instead of being beaded. The infra-orbital line 

 extends below the eye to the ventral surface of the nasal pit. 

 The opercular-mandibular has changed but little from the condi- 

 tion in the previous stage. The post-auditory line has grown 

 craniad so that the anterior end extends to about the middle of the 

 auditory pit. Caudally it extends to a point half way between 

 the operculum and the pectoral fin. This line consists in this 

 stage of three elongated bead-like thickenings placed end to end. 



The connection of the post-auditory line with the infraorbital 

 line shows first in an embryo a day after hatching (Fig. 12). 

 This is the stage with which Allis begins his description. The 

 post-auditory line extends caudally to the pectoral fin and 

 shows a fine beaded appearance. The head lines are very much 

 broader than before and the supra-orbital extends farther over 

 the eye. The infra-orbital sends a small twig up over the 

 nasal pit. The line connecting the infra-orbital with the post- 

 auditory line is a very fine cord. A very small twig is given off 

 from this line just below the auditory organ and projects half 

 way up the caudal border of this organ. There is no differ- 

 entiation of the lines into definite sense organs in this stage. 



Summary. 

 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 in- 

 timate 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. 



