844 IHK NIIRVOUS SYSTLM 



the complicated nasal passageway of the adult frog. The external opening of 

 each passageway is called an external naris while the opening into the buccal 

 cavity is known as the choana (fig. 257B). 



c. Development oj the Olfactory Organs in the Chick 



The development of the olfactory organ in the chick embryo resembles the 

 development of this structure in the mammal, described below. 



(I. Development of the Olfactory Organs in the Mammalian Embryo 



As in other vertebrates, the olfactory areas of the olfactory organs of mam- 

 mals develop from olfactory placodes located one on either side on the ventro- 

 lateral aspect of the primitive head region (fig. 256). The olfactory placodes 

 sink inward to form the olfactory pits, and each pit expands laterally and dis- 

 tally. The lateral external margin of each olfactory pit is called the lateral nasal 

 process, and that of the median external margin is called the median nasal 

 process. The median and lateral nasal processes come in contact with the 

 maxillary process of the upper jaw. 



As the olfactory pit grows posteriad it comes to open into the roof of the 

 primitive oral cavity as the primitive choana (figs. 288A and 256) posterio- 

 medially to the junction of the maxillary and median nasal processes. Later, as 

 each palatal process grows mcdiad from the maxillary processes, the nasal pit 

 and the upper oral area become separated from the oral cavity below by the 

 formation of the secondary palate (fig. 289D-F). Meanwhile, the median 

 nasal septum (fig. 288 A) grows ventrad and posteriad from the fronto-nasal 

 process and unites with the secondary palate in the median line (fig. 288B). 

 Two nasal passageways thus are established leading posteriorly (fig. 288D) 

 to open into the pharyngeal area as the secondary choanac (fig. 289F). The 

 epithelium of the original nasal placode and pit comes to lie in the dorso-medial 

 and dorso-lateral areas of this nasal passageway along either side of the nasal 

 septum (fig. 3591). The olfactory epithelium gives origin to bipolar cells, one 

 pole developing cilia-like processes which lie exposed to the surface of the 

 epithelium while the other pole develops an elongated fiber which grows 

 dorsad and posteriad to enter the forming olfactory bulb of the telencephalon 

 (fig. 356A). 



7. The Eye 



a. General Structure of the Eye 



The general structure of the eye is shown in figure 360A. 



h. Development of the Eye 



The early stages of the development of all vertebrate eyes tend to follow 

 certain generalized steps, and the following description of the developing eye 

 of the chick presents the principles involved. The eye of the chick begins to 



