120 Journal of Comparative Neurology 



the neural ganlion, as is true for all true cranial segmental 

 ganglia." 



Naturally, next, would follow a rapid survey of the pro- 

 gressive differences in general configuration of the peripheral 

 organs, but as this will necessarily be referred to under other 

 topics we will not lengthen the review by introducing a full dis- 

 cussion here. 



2. Evolution of the external and internal nares. — In lower 

 vertebrates the olfactory opening is single. In fishes the olfac- 

 tory sac divides and each comes to have two openings but 

 neither is within the mouth. It would appear that in higher 

 vertebrates the mouth becomes more extensive and envelopes 

 the caudal opening of the nasal sac, producing the internal 

 nares. 



Ganoids and Teleosts have two external openings, anterior 

 and posterior, through which the water passes. 



In Amphibians it is said that the posterior nares are formed 

 in a different manner. The groove closes completely behind 

 and a new perforation is formed from the blind end of the ori- 

 ginal nasal groove. Balfour ("Comparative Embryology," 

 from whom this is condensed) thinks this is a secondary pro- 

 cess. 



Both external and internal orifices are situated far forward 

 near the edge of the mouth in Dipnoi and the amphibian stage. 



In Dipnoi, says Le Conte, we have for the first time 

 nostrils. These fishes completely combine gill-breathing with 

 lung-breathing. The steps from these to the lowest amphibian 

 reptiles is so small, that some have classed the lepidosiren among 

 amphibians instead of fishes. 



In lizards the method of formation of the internal nares is 

 somewhat different. A groove connecting the nasal sac with the 

 roof of the mouth is formed, but it forms on the inner aspect a 

 great lateral diverticle clothed with a continuation of Schneider's 

 epithelium, and forms the fundament of Jacobson's organ. Sub- 

 sequently the groove is closed in and Jacobson's organ separates 

 from the nasal passage and acquires a new opening into the 



