26 CORA J. BECKWITH. 



hyoidean and the first branchial arches. It also probably gives 

 rise to the seventh and ninth cranial nerves. In the region of the 

 cranio-lateral portion of the neural crest there is on each side 

 the auditory invagination of the ectoderm. This becomes con- 

 stricted from the ectoderm and forms a closed vesicle or auditory 

 vesicle. As the auditory vesicle undergoes no further division it is 

 evident that the lateral line anlage does notarise in connection with 

 the auditory organ as described in teleosts. The beginning of the 

 lateral line system appears in a later stage. 



The first indication of a lateral line is found in sections of a stage 

 long after the establishment of the auditory vesicle (Fig. 7) and 

 about a day and a half older than the stage represented by Fig. 5. 

 A thickening of the inner layer of ectoblast caused by the cells 

 becoming columnar runs along each side of the embryo in the 

 angle which the embryo makes with the yolk (Fig. 8, p.a.l.l.). 

 It extends from the second gill slit to the region of the first 

 somite. It is not visible in surface view. This proves to be the 

 anlage of the post-auditory division of the lateral line system. 



In embryos about a day and a half older than that last de- 

 scribed the lateral line is first seen in surface view (Fig. 9, p.a.l.l.). 

 The post-auditory anlage has elongated so as to extend to the 

 middle of the second post-auditory somite. It is a very slender 

 cord formed from the ectodermal thickening described in the pre- 

 vious stage. It is of greater depth than in the earlier stages, so 

 that its outer surface is slightly raised above the surface of the 

 body. The anlage of the head lines also appear in this stage 

 (Fig. 9, s.o.l.l., i.o.l.l., o.m.l.L). The supra-orbital and the oper- 

 cular-mandibular lines form a V, the arms of which extend 

 craniad from a point just in front of the auditory organ. The 

 supra-orbital extends to the caudo-dorsal portion of the eye 

 and consists of two elongated bead-like thickenings end to end. 

 The opercular-mandibular extends down the cranial edge of the 

 gill cover and is a very slender line. Passing from the caudo- 

 ventral portion of the eye and into the angle of the V is the infra- 

 orbital line which is a short broad thickening. The lines in the 

 head region are formed by thickenings of the ectoblast which pro- 

 ject inward very slightly, but also raise the surface in the form 

 of ridges. The thickenings are caused here also by the ecto- 

 derm cells becoming columnar (Fig. 10, i.o. and s.o.l.l. ). 



