Bedford, Olfactory Nerve m Swine. 



397 



is .075 millimeter. From the region of greatest thickness, it 

 gradually thins out in every direction until the ordinary thick- 

 ness of the general epithelium is reached, which is .001 milli- 

 meter. Such a condition of the epithelium is found in a swine 

 embryo of five miUimeters length. Even at this time may be 

 seen a slight indentation in the olfactory plate. In older em- 

 bryos this indentation becomes deeper. This is due to the 

 more rapid growth of the margins of the plate. 



Fig. I. Section through the head of a swine embryo, 5 mm. long, in the 

 plane indicated in the small figure, showing the thickening of the epithelium at 

 future location of olfactory pit. A-B .05 mm., C-D .075 mm., E-F .02 z, mm., 

 G-H .001 mm., K-L .0012 mm., F. B., forebrain, Mes., mesoderm, Ect., 

 ectoderm, 0. PL, olfactory plate. 



As the deepening increases the pit comes to occupy a 

 more ventral position and to lie relatively nearer the mid-line. 

 In an embryo in which there is the earliest appearance of a pit, 

 the distance between the two pits is practically the entire width 

 of the head, while in an embryo of about eleven millimeters in 

 length, the distance between the two pits constitutes only fifty- 

 five percent of the entire width of the head. 



In a young embryo in which the pit is just beginning to 

 form, the axis of the lumen is directed outward at a right angle 



