LATERAL LINE SYSTEM OF AMI A CALVA. 2$ 



vious stage, since the cells are more loosely packed and the cells 

 of the ventral and lateral surfaces merge gradually into the sur- 

 rounding mesoblast (Fig. 4, n.c). On account of this pushing 

 apart of the cells the crest extends farther ventrally along the 

 sides of the neural tube. Laterally it extends out on either 

 side to a distance equal to the width of the neural tube. The 

 auditory invagination lies between the two lobes of the crest. A 

 cavity has appeared in the auditory invagination in the form 

 of a slit separating the, two walls along the plane spoken of in 

 the previous stage. A constriction separating the invagination 

 from the ectoderm is in process of formation. 



In a stage in which two gill slits (spiracular and first post- 

 hyoidean) are formed (Fig. 5) the two lobes of the neural crest 

 seen in surface view of the previous stage are no longer apparent 

 in surface view. Sections show that the lobes of the neural crest 

 have extended further laterally and at the same time thinned out 

 dorso-ventrally so that they are transformed into two narrow 

 bands of mesoblast extending outward and forward from the 

 neural tube, one into the hyoid arch and the other into the first 

 branchial arch. The auditory invagination lies at the median 

 end of the post-hyoidean slit between the ends of the hyoid and 

 branchial arches and extending into the mesoblast of the pos- 

 terior edge of the hyoid arch. The constriction observed in the 

 last stage between the ectoderm and the auditory invagination 

 has proceeded until the auditory invagination is now a closed ves- 

 icle nearly separated from the ectoderm (Figs. 5, a.p. ; 6, a.m.). 



This auditory invagination was traced in sections of later stages 

 until in a stage like Fig. 7 it has become a large vesicle, circular 

 in section, lying some distance below the ectoderm. Its median 

 half is partly covered by the hind brain so that only the lateral 

 portion is visible in surface view. Its wall is composed of a 

 single layer of cells which are more columnar on the median 

 side than on the lateral. 



The thickening on the neural tube proves then to be a mass 

 of mes-ectoblast in the form of a neural crest in which the audi- 

 tory invagination lies and so cannot be the anlage of the lateral 

 line system. The neural crest of this region divides into two 

 lobes which go in large part to form the mesodermal portion of the 



