STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 771 



has its parallel in the brain of Protopterus, and as pointing in 

 the same direction a general similarity in the appearance of the 

 brain of Polypterus to that of the Dipnoi may be mentioned. 



There appears to us to be in no points a close resemblance 

 between the brain of Ganoids and that of Elasmobranchii. 



SENSE ORGANS. 



Olfactory organ. 



Development. The nasal sacks first arise during the late em- 

 bryonic period in the form of a pair of thickened patches of the 

 nervous layer of the epiblast on the dorsal surface of the front 

 end of the head (Plate 37, fig. 39, ol.). The patches very soon 

 become partially invaginated ; and a small cavity is developed 

 between them and the epidermic layer of the epiblast (Plate 37, 

 figs. 42 and 43, ol.}. Subsequently, the roof of this space, formed 

 by the epidermic layer of the epiblast, is either broken through 

 or absorbed ; and thus open pits, lined entirely by tlie nervous 

 layer of tlic epidermis, are formed. 



We are not acquainted with any description of an exactly 

 similar mode of origin of the olfactory pits, though the process 

 is almost identical with that of the other sense organs. 



We have not worked out in detail the mode of formation of 

 the double openings of the olfactory pits, but there can be but 

 little doubt that it is caused by the division of the single open- 

 ing into two. 



The olfactory nerve is formed very early (Plate 37, fig. 39, I), 

 and, as Marshall has found in Aves and Elasmobranchii, it 

 arises at a stage prior to the first differentiation of an olfactory 

 bulb as a special lobe of the brain. 



The Eye. 



Anatomy. We have not made a careful histological examin- 

 ation of the eye of Lcpidostcns, which in our specimens was not 

 sufficiently well preserved for such a purpose ; but we have 



