OLFA CTOR Y ORG A NS. 



441 



senting the process in a very simple form. The general history of 

 the process was first made out by KoUiker. 



<^«f ma 



y.ao 



Fig. 310. Diagrammatic vektical section thkougu the head of a larva of 



Petkomyzon. 



The larva Lad been hatched three days, and was 4 "8 mm. in length. The optic and 

 auditory vesicles are supposed to be seen through the tissues. 



c.h. cerebral hemisphere ; th. optic thalamus ; r«. infundibulum ; ^j?!. pineal gland ; 

 vih. mid-brain ; ch. cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; of. 

 optic vesicle ; ol. olfactory pit ; m. mouth ; hr.c. branchial pouches ; th. thyroid in- 

 volution ; v.ao. ventral aorta ; lit. ventricle of heart ; ch. notochord. 



The opening of the nasal pit becomes surrounded by a ridge except 

 on its oral side. The deficiency of this ridge on the side of the mouth 

 gives rise to a kind of shallow groove leading from the nasal pit to 

 the mouth. Tlie ridge enveloping the opening of the nasal pit next 

 becomes prolonged along the sides of this groove, especially on its 

 inner one ; and at the same time the superior maxillary process grows 

 forwards so as to bound the lower part of its outer side. The inner 

 and outer ridges, together with the superior maxillary process, enclose 

 a deep groove, connecting the original opening of the nasal pit with 

 the mouth. The process just described is illustrated by fig. 311 A, 

 and it may be seen that the ridge on the inner side of the groove 

 forms the edge of the fron to-nasal process {h). 



On the sixth day (Born, 394) the sides of this groove unite together 

 in the middle, and convert it into a canal open at both ends — the 

 ventral openings of the canals of the two sides being placed just within 

 the border of the mouth, and forming the posterior nares ; while the 

 external openings form the anterior nares. The upper part of the 

 canal, together with the original nasal pit, is alone lined by olfactory 

 epithelium; the remaining epithelium of the nasal cavity being indif- 

 ferent epiblastic epithelium. Further changes subsequently take 

 place in connection with the posterior nares, but these are described 

 in the section dealing with the mouth. 



In Mammalia the general formation of the anterior and posterior 

 nares is the same as in Birds; but, as shewn by Dursy and Kolliker, 

 an outgrowth from the inner side of the canal between the two open- 



