466 Journal of Comparative Neurology and Psychology. 



Fig. 4, represents a transverse section of an 1 8-day embryo of Necturus 

 through the primary optic vesicles, which are now well defined. The neural folds 

 have closed, and the superficial epiblast has separated from the neural epiblast. 

 The cellular contents of the neural canal are less compact, indicating the begin- 

 ning of cell disintegration in the median portion. 



Fig. J, represents a transverse section of an i8 l-2-day embryo of Necturus 

 passing through the primary optic vesicles, whose walls are now thinned to a 

 single layer of cells, as shown in the optic vesicle on the right. Cellular disinte- 

 gration has taken place at the center of the neural canal, leaving a well defined 

 clear space. This condition represents an advance beyond that shown in Fig. 4. 



Fig. 6, represents a section, intermediate between horizontal and transverse, 

 of a 20-day embryo of Necturus. The optic vesicles are well advanced, showing 

 a distinct evagination and backward growth. At this time the lumen is entirely 

 free from cells or their remains. 



Figs. 7-iJ, illustrate the cytologic changes in the cytoplasm and nuclei of the 

 cells which take part in the formation of the walls and cavities of the primary 

 optic vesicles. 



