39^ Journal of Comparative Neurology and Psychology. 



to the lateral aspects of the head. As the embryo becomes 

 older the axis of the lumen gradually shifts until it is directed 

 ventrally, having passed through an angle of nearly 90 degrees. 



Up to the time of the first appearance of the nerve the 

 pit has the form of a simple invagination. No folds as yet 

 have appeared in its walls. Thus a well-formed pit has devel- 

 oped before there is any indication of the olfactory nerve. 



Differentiation of the Cells of the Olfactory Epithelium. — 

 In embryos possessing an olfactory plate in which only a very 

 slight indication of an invagination appears, two kinds of cells 

 may be seen in the epithelum ; first, the ordinary prismatic 

 cells ; second, large cells almost spherical in shape, whose cyto- 

 plasm is not readily stained. The latter are usually located in 

 groups of three or four. In embryos of this stage of develop- 

 ment they are always situated near the external margin of the 

 epithelium. In an embryo of six millimeters, in which the dif- 



W, 



i\Wi 



im\\ p 





/^tg. 2. Sections of olfactory epithelium showing germinating cells, G. C, 

 located near lumen of the nasal pit ; L. P., Lumen of Pit. Iron haematoxylin . 

 A, from an embryo 9 millimeters in length ; B, from an embryo 6 millimeters in 

 length. X 533- 



ferentiation of the cells was first observed, a number of these 

 cells show different stages of karyokinetic division. The nuclei 



