264 



THE DEVELOPMENT OF THE CHICK 



from the central ends of the neiirol^hxsts and grow into the 

 mesenchyme towards the fore-brain. At the same time groups 

 of epitheUal cells free themselves from the inner face of the 

 olfactory epithelium, and come to lie between this and the fore- 

 brain. The axones of the neuroblasts grow between these cells 

 until they reach the base of the fore-brain over which they spread 

 out, entering the olfactory lobe about the sixth day (Figs. 155 

 and 156). In the meantime the peripheral ends of the olfactory 

 neuroblasts have extended out as broad protoplasmic processes 

 to the surface of the olfactory epithelium, and thus form the per- 

 cipient part of the olfactory sense-cells. 



Fig. 155. — Olfactory epithelium of a chick embryo of 5 

 days, prepared by the method of Golgi. (After Disse.) 

 a, b, and c indicate different forms of neuroblasts in the 

 olfactory epithelium. 



The epithelial cells between fore-brain and olfactory pit, through 

 which the axones of the olfactory neuroblasts grow, are for the 

 most part supporting and sheath-cells of the nerve, but they in- 

 clude a few bipolar neuroblasts (Fig. 156). The latter are to 

 be considered as olfactory neuroblasts with elongated protoplas- 

 mic processes. 



Rubaschkin finds a ganglion, which he calls ganglion olfactorium 

 nervi trigemini, situated beneath the olfactory epithelium in a nine- 

 day chick. The bipolar cells send out processes peripherally which end 

 in fine branches between the cells of the olfactory mucous membrane, 

 and centrally, which go by way of the ramus olfactorius nervi 

 trigemini towards the Gasserian ganglion. 



2. The Secoyid Cranial or Optic ■ Nerve. The course of this 



