OLFACTORY ORGANS. 



443 



and assists in forming its under wall. This ingrowth of the superior 

 maxillary process is the rudiment of the hard palate. 



Ou the conversion of the nasal groove into a closed passage, the 

 opening of Jacobson's oi'gau into the groove 

 becomes concealed ; and at a later period 

 Jacobson's organ becomes completely shut 

 cfF from the nasal cavity, and opens into the 

 mouth at the front end of an elongated 

 groove leading back to the posterior nares. 



In Amphibia the posterior nares are 

 formed in a manner very different from that 

 of the Amniota. At an early st;ige a shallow 

 groove is formed leading from the nasal pit 

 to the mouth ; but this groove instead of 

 forming the posterior nares soon, vanishes, 

 and by the growth of the front of the head 

 the nasal pits are carried farther away from 

 the mouth. 



The actual posterior nares are formed 

 by a perforation, in the palate, opening into 

 the blind end of the original nasal pit. 



Considering that the various stages in the 

 formation of the posterior nares of the Am- 

 niota are so many repetitions of the adult 

 states of lower forms, it may probably be 

 assumed that the mode of formation of the posterior nares in Amphibia is 

 secondary, as compared with that in the Amniota. 



A diverticulum of the front part of the nasal cavity of the Anura is 

 probably to be regarded as a rudimentary form of Jacobson's organ. 



Fig. 312. Section through 



THE NASAL CAVITY AND JaCOBSON's 



ORGAN. (From Gegenbaur.) 



sn. septum nasi ; en. nasal 

 cavity ; J. Jacobson's organ ; d. 

 edge of upper jaw. 



Bibliography. 



(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbel- 

 thiere." Parts i. and 11. Morphologisches Jahrbuch, Bd. v., 1879. 



(395) A. Kolliker. " Ueber die Jacobson'schen Organe des Menschen." Fest- 

 schnft f. Rienecker, 1877. 



(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ." 

 Quart. Journ. of Micr. Science, Vol. xix., 1879. 



Se7ise organs of the lateral line. 



Although I do not propose dealing with the general development of 

 various sense organs of the skin, there is one set of organs, viz. that of the 

 lateral line, which, both from its wide extension amongst the Tchthyopsida 

 and from the similarity of some of its parts to certain organs found 

 amongst the Chtetopoda', has a great morphological importance. 



' The organs which resemble those of the lateral line are the remarkable sense 

 organs found by Eisig in the Capitellidae {Mittheil. a. d. Zool. Station zu Neapel, Vol. i.); 

 but I am not inclined to think that there is a true homology between these organs and 

 the lateral line of Vertebrata. It seems to me probable that the segmentally arranged 

 optic organs of Polyophthalmus are a special modification of the more indifferent 

 sense organs of the Capitellidae. The close affinity of these two types of Chaetopods is 

 favourable to this view. 



