102 F. W. STEWART 



cells occurring along the ophthalmic division of the trigeminus. 

 The wi-iter has attempted in the present study to follow out the 

 proliferations from the olfactory epithelium and at the same time 

 to seek for possible evidences of a spreading-in of cells from other 

 sources, namely, 1) as forward extensions from the cervical 

 region, that is, growth forward along the internal carotid nerve; 

 2) migrations along trigeminus branches, and, lastly, 3) elements 

 possibly entering from the sphenopalatine ganglion. 



Individuals of a litter present considerable variation, thus 

 making it possible at one age to secure several stages. In 

 general it may be said that the earliest proliferations from the 

 olfactory epithelium, in the albino rat, occur between the ages 

 12 days 6 hours and 12 days 6^ hours after insemination. I 

 have but one embryo of the 12-day-6-hour litter sectioned. In 

 this embryo the olfactory epithelium is still in the plate stage. 

 It consists in its thickest part of some seven layers of nuclei. 

 Certain of its cells possess an elongate," spindle-shaped process 

 directed centrally, but in no instance has the process been traced 

 beyond the limits of the epithelium. The mesenchyme shows a 

 marked tendency to condense beneath the epithelium and an 

 abundant vascular network is present. Mitotic figures are 

 numerous in the epithelium. At this stage the ramus ophthal- 

 micus V contains large numbers of fusiform neuroblasts situated 

 en masse from the Gasserian ganghon to the posterior border of 

 the optic stalk. The nuclei are elongate and the technique 

 permits the following of the processes a distance of some four or 

 five times that of the nuclear length in either direction. The 

 cells situated farthest out on the ramus ophthalmicus resemble 

 in every way those on the Gasserian ganglion. A description 

 of the early ramus ophthalmicus has been given in an earlier 

 paper ('20) under the consideration of the ciliary ganglion. 



In the earliest stage encountered in the 12-day-6^-hour litter, 

 the olfactory plate has become transformed into an exceedingly 

 shallow pit. Its thickened epithelium is traceable through 

 thirty-six sections (sagittal). In this embryo the first evidence 

 of proliferations giving rise to cells which leave the epithelium 

 is noted. The zone of proliferation is first met with six sections 



