240 NERVE COMPONENTS OF BONY FISHES. 



the largest being half the size of the largest naked cuta- 

 neous sense organs found on the outer surface of the head. 



Passing caudad, shortly before reaching the posterior 

 nasal opening the lamellae disappear and with them the 

 sensory epithelium, the entire sac from this point caudad 

 being lined by ordinary thin pavement epithelium. The 

 sac narrows in its transverse diameter and at the same 

 time becomes much deeper. It extends as a rather narrow 

 cleft so far ventrally that its deepest point lies in the same 

 horizontal plane as the dorsal ends of the dorsal diverticula 

 from the lateral edge of the pharyngeal roof, these diver- 

 ticula lying somewhat laterally of the olfactory sac. The 

 outline of the nasal sac is drawn on Fig. 5, the sensory 

 portion of the wall being shaded. The posterior nasal 

 aperture is a long narrow slit four or five times the length 

 of the anterior. The nasal sac extends only a very short 

 distance caudad of it. For further notes on the conforma- 

 tion of the olfactory fossa see the account of the terminal 

 portion of the supra-orbital trunk. Section 7, IX. 



The development of the olfactory organ has been 

 worked out in a series of post-embr^^onic stages and I 

 fully confirm Madrid-Moreno ('86) that the ontogeny dis- 

 proves the elaborate assumptions of Blaue ('84) that the 

 olfactory epithelium is derived from a bud of the lateral 

 line system which has wandered into the olfactory fossa, 

 there multiplied to form the system of " olfactor)^ bads" 

 of the teleosts and then, in most higher forms, formed 

 secondarily a continuous sensory surface by the fusion of 

 the buds. 



From the standpoint of theoretical morphology alone 

 Blaue's results could not stand; but, as it is a question of 

 no small importance to the fundamental head problems, 

 and as Blaue's errors reappear in recent editions of several 



