THE CRANIAL GANGLIA IN AMEIURUS 327 



As the neural keel deepens and assumes the form of a cord (fig. 

 C), and as the blastoderm rises on the yolk and assumes a rounded 

 form, the lateral cell masses are brought gradually into a lateral 

 position, still retaining their connection with the dorsal half of 

 the cord by an intermediate slightly constricted area. 



In an embryo in which the optic vesicle has reached a stage 

 in which the future optic cup is slightly larger than the stalk (Stage 

 II), this lateral thickening begins some five or six sections pos- 

 terior to the stalk and extends from this point back beyond the 

 region in which the Xth ganglion is later formed, more than one 

 hundred sections. Posterior to this point where the keel is form- 

 ing in the region of the spinal cord it gradually becomes reduced 

 in size as the keel becomes shallower. Throughout this whole 

 region the lateral cell mass has a structure quite uniform at first, 

 varying only in shape, being somewhat thicker and more closely 

 applied to the sides of the brain in the anterior region (fig. 2), 

 and somewhat thinner and more dorsally attached to the cord in 

 the posterior region, particularly posterior to the position in which 

 the auditory vesicle develops. 



Six sections posterior to the optic vesicle (fig. 2) , the lateral 

 mass is applied to the dorsal half of the cord and is homogeneous 

 in structure. This section lies in the region anterior to that in 

 which the Gasserian ganglion later appears. In the region in 

 which the Gasserian ganglion forms and posterior to it (fig. 3) , 

 the lateral mass is broader and thinner and the attachment to 

 the neural cord is less extensive. This condition of the lateral mass 

 persists throughout the region in which the Gasserian ganglion 

 forms and back of this until we come to the region just anterior 

 (fig. 4) and just posterior (fig. 5) to the auditory vesicle, where, 

 in a slightly older embryo (Stage III), there is soon noticeable 

 a slight differentiation of the lateral mass into a thicker dorsal 

 portion, the dorso-lateral mass (D. L. M., figs. 4 and 5), connected 

 with the cord by the intermediate region, and a ventral mass 

 (Pre. PL, fig. 4, Post. PL, 5) slightly separated from this dorsal 

 mass on its mesial border by a constriction. This ventrally 

 differentiated mass, or dorso-lateral placode (pre. au. placode, 

 fig. D) is present throughout the whole auditory region (fig. 6) J 



