THE CRANIAL GANGLIA IN AMEIURUS 323 



than the youngest of the A. melas series, which was forty-nine 

 hours old. In the following brief characterization of the stages 

 the optic cup, the optic lens, and the auditory vesicle are used 

 chiefly as a means of separating them in addition to the more 

 minute differences to which attention is called in the body of the 

 paper. 



Stage I. An early stage (24 hours?) of A. nebulosus in which 

 the blastoderm is nearly flat, the neural keel being elevated slightly 

 above the yolk at the anterior end only. The blastoderm has not 

 extended sufficiently far posteriorly and ventrally to ' be cut on 

 the ventral side of the yolk sac at the posterior end of the embryo. 

 Fig. 1 was drawn from this stage. 



Stage II. Optic vesicle present, but solid. Nuclei of cells in 

 optic vesicle uniformly distributed with no indication of the peri- 

 pheral arrangement of the nuclei which precedes the formation of 

 a cavity. Nuclei of auditory vesicle irregularly arranged with no 

 indication of a cavity. The auditory vesicle can be located only 

 by comparison with an older series. Lateral mass not broken 

 down into mesectoderm at any point. Figs. 2 and 3 are drawn from 

 this stage. 



Stage III. Optic vesicle solid but with nuclei of cells uniformly 

 arranged around the periphery of the vesicle preparatory to the 

 formation of the optic cup. Auditory vesicle with its cells elon- 

 gated and the nuclei arranged around the periphery. The lateral 

 walls of the cup not in contact, the future cavity of the cup filled 

 with spherical cells. The solid lateral mass of the preceding stage 

 changed into a loose mass of tissue in the region of the Gasserian 

 ganglion, particularly just anterior and posterior to this point. 

 The pre- and postauditory placodes are present. Figs. 4, 5, 6, 

 7, 8, 9 were drawn from this stage. 



Stage IV. Optic vesicle with slight cavity. Epidermis not 

 thickened preparatory to the formation of a lens. Auditory vesi- 

 cle with a slight cavity and the lateral walls of the vesicle in con- 

 tact. Preauditory placode just detached fom the auditory vesicle. 

 Postauditory placode still in contact with the auditory vesicle. 

 Gasserian and geniculate ganglia not sufficiently well defined to 

 determine approximately their boundaries. Hyoid gill pocket 



