Morphological Studies. 757 



Born writes as follows: — „Bei ganz jungen Embryonen sah ich die 

 Mündung des JACOBSON'schen Organes so sehr nach vorn gewendet, 

 dass sie mehr neben als in der Oeffnung der Riechgrube erschien, es 

 liegt dies die Frage nahe, ob dasselbe aus der Riechgrube ausgestülpt 

 werde oder zugleich mit und neben dieser entstehe, und erst später 

 in dieselbe eingesenkt werde" (4, p. 79). 



And Fleischer, in describing his researches among Reptiles, says : 

 „In den frühesten Stadien, die zur Untersuchung vorgelegen haben, ist 

 bereits ein vom Nasenhöhlenepithel ausgehendes Divertikel nachweisbar, 

 wie wir bei den Säugern beobachtet haben. Nur ist dasselbe viel 

 grösser als das bei letzteren beschriebene. Von einem Epithel ausge- 

 kleidet, nimmt es fast die Hälfte der ganzen Nasenhöhle ein." 



Indeed, examination of the literature of Jacobson's organ shows 

 clearly that just in the group of animals, the Reptilia, in which alone 

 it can be considered as something more than a rudimentary organ, a 

 thorough comparative investigation is a desideratum. 



Of the structure of Jacobson's organ we have many researches 

 by Klein (16 to 19), Leydig (22, 23), Wright (34), and Born (3, 4, 

 5, 6) : while Born and Solger (31) have given good accounts of the 

 anatomical relationships of the organ and its position in the cranium. 

 Hence in the following pages only the most important points of the 

 nature and origin of Jacobson's organ will be considered. For this 

 purpose its earlj development will be described in connection with the 

 olfactory nerve of Reptiles, and following this, some points in the em- 

 bryonic and adult structure which have important bearings on the 

 question of the morphology of the nose will be discussed. 



Material. 



The investigations were made on a great number of embryos of 

 Lacerta agilis and Anguis fragilis, and also on some half a dozen 

 typical stages of Tropidonotus natrix. 



For the olfactory nerve I was able to make use of the large 

 series of sections made for a recent study (37) of the peripheral 

 nervous system of Vertebrates, and have thereby been able to correct, 

 confirm, and extend my former statements (36, pp. 33 to 42). 



The deyelopment of the olfactory organ and nerve 

 in Elasmohranchii. 



The first account of the development of the olfactory nerve of 

 Elasmobranchs is that of Marshall (52, 53). He concludes that the 



5U* 



