Morphological Studies. 755 



organ, a series of very complicated sense organs, the cochlea, semi- 

 circular canals &c. , has been evolved. All, as can be shewn deve- 

 lopmentally, by the growth and differentiation of one small piece of 

 sensory epithelium. 



Another case may be instanced of the formation of sense organs 

 in Vertebrates from other sense organs. It is a well known fact, 

 originally noticed by Malbranc (51), and afterwards confirmed by 

 SoLGER, myself and others, that the lateral sense organs multiply by 

 division. All these facts appear to go a long way towards establishing 

 the dictum that Vertebrates cannot form new sense organs out of in- 

 diffèrent cells. 



Whence then is the Vertebrate olfactory organ derived? Mar- 

 shall (52) and Dohrn (39) have supposed from a gill-cleft. The 

 position taken up in this and some earlier memoirs by me is that it 

 is a higher differentiation of a preexisting segmental lateral or bran- 

 chial sense organ, just as Jacobson'» organ is also a further deriva- 

 tive and differentiation of the olfactory sense organ, which was there 

 before it. 



Besides the researches on the olfactory organ proper it is pro- 

 posed to give some account of the development and minute structure 

 of Jacobson's organ in lizards and snakes. 



In this account some very interesting facts will be recorded, 

 facts which shew that very little is known about this organ, although 

 it has been investigated in Reptilia by Leydig, Fleischer, Solger 

 and Born. 



History of Jacobson's organ. 



Since the discovery of this organ at the beginning of the present 

 century by the Danish anatomist whose name it bears (15), it has 

 been investigated by many morphologists. Though the literature is 

 not very extensive, it is rather difficult to collect together, and there 

 are one or two memoirs in the literature cited, which I have not been 

 able to obtain. They are marked by an asterisk. 



One of the earliest investigators, Eathke, describes his researches 

 as follows (29, p. 88): „Noch ehe die Bildung einer Nasenhöhle be- 

 werkstelligt ist, entsteht auch schon die den Schlangen eigenthümliche 

 Nasendrüse. Unfern des künftigen unteren Nasenloches bildet sich 

 ungefähr um die Mitte dieser Periode aus der Riechhaut, wenn diese 

 schon zu einer Mulde zusammengebogen ist, und zwar aus der gegen 

 die Mittellinie des Kopfes zugekehrten Hälfte derselben, eine Ausstülpung, 



Zool. Jahrb. III. Abth. f. Morph. r^^J 



