to five in the case of Beard. The current view is that there 

 are three corresponding to the three somites. 



The neuromeres observed in the development of the brain 

 have been used in this connection; their number is presumed 

 to equal the number of somites. However, the cranial nerves 

 do not have a simple one-to-one relationship with these 

 (for example, the abducens of the shark has its nucleus in 

 neuromeres 5 to 7). Further, neuromeres are more conspic- 

 uous in the higher forms than in the lower forms. At best it 

 is difficult to associate these with other structures. 



In reviewing the materials on the conducting portion of 

 the nervous system, agreement among the gnathostomes is 

 marked while the variation between the two cyclostomes is 

 exceptional. The case of the agnaths can be accounted for 

 on the basis of the great length of time assumed to separate 

 these two branches of the agnath fishes, a period of sepa- 

 ration which may exceed the total length of time involved in 

 the radiation of the gnathostome fishes. On the basis of a.gree- 

 ment of agnath and gnathostome types of brains, we can as- 

 sume that five divisions, or six if one choses to identify the 

 olfactory bulbs as a separate one, were established before 

 separation of these two lines. Similarily the formation of the 

 multisegmental fifth, seventh, and tenth nerves preceded this 

 separation. 



SENSORY ORGANS 



The nervous system is associated with a number of highly 

 developed organs of which the nose, eye, and ear are the 

 most obvious. Less obvious are the Jacobson's organ associ- 

 ated with the nose, the lateral-line system of the fishes 

 which is closely related in function to the ear, and the vari- 

 ous chemical senses, which may be related to the nose. 

 Organs capable of temperature evaluation are present in 

 some animals. Within the body are other sensory structures 

 used in proprioception. These usually involve single cells or 

 aggregations of cells around a sensory ending. 



Nasal structure in tetrapods 



Bilateral nasal structures are developed in all of the ver- 

 tebrates. In the cyclostome fishes there is a single midline 

 nasal sac in the adult. This arises from a single placode in 

 which bilateral invaginations appear and then join. Two ol- 

 factory nerves, as areas of fibers, also indicate the bilateral 

 nature of this structure. 



The nasal organ arises in embryology from an epidermal 

 placode on either side which invaginates into the snout to 

 form a vesicle. This has a single opening to the exterior. 

 The single opening to the exterior is variously divided into 

 external and in some cases internal nasal openings. 



It has been suggested that the reptile represents the more 

 primitive style of nasal passage observed in living forms of 

 tetrapods. In this group the nasal sac becomes elongated to 

 form a groove, one end of which opens on the outside of the 



mouth margin and the other on the inside of the mouth (Fig- 

 ure 13-19). The margins of this groove meet in the middle 

 section to form the jaw margin and separate an external 

 and an internal nares. 



In the mammal the course of events is somewhat dif- 

 ferent; the groove closes throughout its posteroinner half, 

 leaving only an external naris. This closure produces a buc- 

 conasal (oro-nasal) membrane in the position of the internal 

 naris which later ruptures. In the Echidna, development is 

 reported to be as in the reptile. 



In certain amphibians the posterointernal half of the 

 groove closes and becomes a strand of tissue, with or with- 

 out a lumen. If the lumen is present, it secondarily comes to 

 open into the mouth as the internal naris; if the lumen is 

 not present, a cavity develops from the outer end of the 

 strand into the mouth, thus secondarily producing the in- 

 ternal naris. Whether the development is direct from a 

 groove or whether it involves secondary opening into the 

 mouth does not seem to be particularly important, although 

 it may be interpreted as two quite different ways of devel- 

 opment. 



The origin of the lacrimal duct in tetrapods appears to be 

 much alike in the different groups. A nasoptic furrow ex- 

 tends from the nasal opening to the eye, but the nasolacrimal 

 duct is not formed directly from this structure. In man it 

 appears first at the eye and extends along the course of the 

 furrow to the nasal capsule. In the frog the middle part of 

 the duct appears first as a solid strand which extends and 

 tubulates in either direction. It is probable that originally 

 the nasoptic groove gave rise directly to the lacrimal duct. 

 It has been suggested that the dipnoans are basically dif- 

 ferent from the amphibians in their nasal development. Their 

 nasal openings have been compared with the two external 

 openings of the osteolepiform crossopterygian rather than 

 the external and internal openings of the tetrapods. The en- 

 tire question of the evolution of the upper jaw of the dip- 

 noan must be more thoroughly understood before its nasal 

 development can be properly assessed. 



In the actinopterygian the opening of the vesicle is di- 

 vided into anterior and posterior outer openings. The anterior 

 one is valved so that water flows into it, through the cham- 

 ber and across the olfactory organ, then out the posterior 

 opening. Water flow is produced by forward motion of the 

 fish, by action of the ciliated lining of the capsule, or by 

 compression of the nasal chamber with respiratory move- 

 ments. 



In the shark there is a single opening partly divided by a 

 flap into anterior and posterior openings. In the rays and 

 Chimaera the flap leads from the nasal opening back to the 

 mouth (Figure 13-29). 



In the wall of the nasal passage of the mammals, turbi- 

 nals develop and there is a restricted area of olfactory 

 epithelium dorsally. The cells of the sensory epithelium give 

 rise to fibers which extend into the olfactory bulb of the 

 brain where they synapse with olfactory tract neurons. This 

 general picture applies to amniotes generally. The Amphibia 



402 



THE NERVOUS SYSTEM 



