24 CORA J. BECKW1TH. 



In embryos in which the optic vesicles and hind brain are just 

 being differentiated from the neural tube there is seen in surface 

 view a thickening (Fig. i, th.) on the neural tube just back of the 

 hind brain. This thickening appears to be a lateral extension of 

 either side of the neural tube and is very similar in appearance to 

 the optic vesicles. It was thought that this might be the anlage 

 for the lateral line. This view was made more probable by the 

 fact that in a later stage (Fig. 3, th.), when the first gill slit 

 (spiracular) is formed, this thickening becomes divided on either 

 side by a transverse fissure into two lobes, the cranial one of 

 which lies opposite the hyoid arch. This bilobed condition sug- 

 gests the division spoken of by Wilson in teleosts. 



Sections (Fig. 2, n.c.) through the region of this thickening 

 in the first stage described (i. e., Fig. 1) show the neural tube still 

 connected with the ectoderm while a wedge-shaped mass of 

 closely packed cells, or neural crest, lies on either side of the tube 

 between the ectoderm and the dorsal surface of the tube. Later- 

 ally these wedge-shaped masses project out beyond the neural 

 tube. The lines of separation between this very much thickened 

 neural crest and the neural tube on one hand and the ectoderm 

 on the other are very indistinct. This neural crest corresponds 

 exactly in position and extent to the external thickening described 

 for the stage shown in Fig. 1. 



In sections through the anterior half there is on either side of 

 the crest the deep auditory invagination of the inner layer of ecto- 

 derm. The invagination is directed ventrally and inward toward 

 the neural tube. The medial side lies against the broad end of 

 the wedge-shaped neural crest. The wall of the auditory invagi- 

 nation consists of a single layer of columnar cells. The two 

 walls are separated from each other by a plane extending from 

 the outer layer of ectoderm to the inner surface of the invagina- 

 tion (Fig. 2, a.m.). Sections toward the anterior end and middle 

 of the crest show that the auditory invagination ends abruptly in 

 both cranially and caudally. 



In a little older embryo (Fig. 3) sections show that the neural 

 tube has lost its connection with the ectoderm. The neural crest 

 has become divided into an anterior and posterior lobe as de- 

 scribed in surface view and is not so clearly defined as in the pre- 



