OLFACTORY ORGANS. 



439 



become very pronounced (fig. 309), serving greatly to increase the 

 surface of the olfactory epi- 

 thelium. At a very early f^ -7^ "'"'^ 

 stage the olfactory nerve 

 attaches itself to the ol- 

 factory epithelium. 



In Petromyzon the ol- 

 factory organ arises as an 

 unpaired thickening of the 

 epiblast, which in the just 

 hatched larva forms a shallow 

 pit, on the ventral side of 

 the head, immediately in 

 front of the mouth. This 

 pit rapidly deepens, and soon 

 extends itself backwards 

 nearly as far as the infun- 

 dibulum (fig. 310, ol). By 

 the development of the upper 

 lip the opening of the olfac- 

 tory pit is gradually carried 

 to the dorsal surface of the 

 head, and becomes at the 

 same time narrowed and 

 ciliated (fig. 47, ol). The 

 whole organ forms an elon- 

 gated sack, and in later 

 stages becomes nearly divi- 

 ded by a median fold into 

 two halves. 



It is probable that the 

 unpaired condition of the 

 olfactory organ in the Lam- 

 prey has arisen from the 

 fusion of two pits into one ; 

 there is however no evidence 

 of this in the early develop- 

 ment; but the division of 

 the sack into two halves by 



a median fold may be regarded as an indication of such a paired character 

 in the later stages. 



In Myxine the olfactory organ communicates with the mouth through 

 the palate, but the meaning of this communication, which does not appear 

 to be of the same nature as the communication between the olfactory pits 

 and the mouth by the posterior nares in the higher types, is not known. 



The opening of the olfactory pit does not retain its embryonic 

 characters. In Elasmobranchii and Chimara it becomes enclosed by 

 a wall of integument, often deficient on the side of the mouth, so that 

 there is formed a groove leading from the nasal pit towards the angle of 

 the mouth. This groove is usually constricted in the middle, and the 



af 



Fig. 307. Views of the head of Elasmo- 

 beanch embryos at two stages as transparent 



OBJECTS. 



A. Pristiurus embryo of the same stage as fig. 

 28 F. 



B. Somewhat older Scyllium embryo. 



III. third nerve; V. fifth nerve; VII. seventh 

 nerve; au.n. auditory nerve; gl. glossopharyngeal 

 nerve ; Vg. vagus nerve ; fb. fore-brain ; pn. pineal 

 gland; mb. mid-brain; lib. hind-brain; iv.v. fourth 

 ventricle; cb. cerebellum; ol. olfactory pit; op. 

 eye; au.V. auditory vesicle; m. mesoblast at base 

 of brain; c/t. notochord; 7i<. heart; Fc. visceral 

 clefts; eg. external gills; pp. sections of body- 

 cavity in the head. 



