through the same mechanics observed in many invertebrates. 

 This is a reaction which cannot as yet be explained. 



The development of a median eye from the epiphyseal 

 outgrowth of the diencephalon was the result of parallel 

 events or interactions of brain tissue with hght when brought 

 close to the skin. In this case the "lens" is of neural origin 

 rather than ectodermal. Whether the median organ (or 

 organs) ever was an eye is doubtful since it finds its highest 

 development in the reptile not in the fishes. As a photosen- 

 sory organ (not image forming) it is useful in diurnal 

 rhythm responses and in chromatophore control (color 

 changes). On the basis of the presence of a parietal foramen, 

 an organ was developed in the most primitive vertebrates, the 

 ancestors of both agnath and gnathostome. 



The ear 



The ear serves two basic functions: equilibrium and hear- 

 ing. Equilibrium involves responses to gravity as well as 

 angular acceleration produced by movement. The term 

 hearing is difficult to define. Vibrations in the earth or in 

 solid structures can be detected by the organs of touch, and 

 in effect this is hearing. Hearing is generally reserved for 

 vibrations of higher frequency and lesser magnitudes. Be- 

 cause of the low energy levels involved, special mechanisms 

 are necessary. In the tetrapods, the inner ear is associated 

 with a middle-ear cavity and tympanum. Connecting the 

 inner ear and tympanum, across the cavity, is a compound 

 columella (bird, reptile, amphibian) or a series of three bony 

 ossicles (mammals). 



The tefrapod ear The inner ear arises from a thickened 

 ectodermal area, the auditory placode, which invaginates 

 to form the otic vesicle. The development of this vesicle 

 varies somewhat, but a general pattern applies to most ver- 

 tebrates. In the Bullfrog, the vesicle, or otocyst, is divided 

 by ridges into a medioventral endolymphatic duct and the 

 inner ear proper. The latter develops evaginated folds for 

 the three semicircular canals; ventrally an outpocketmg 

 forms the sacculus. With completion of separation of the 

 middle parts of the semicircular canals, the utriculus is de- 

 fined. Three lesser diverticula, the basilar and amphibian 

 papillae and the lagena, separate from the sacculus (Figure 

 13-20). All of these become more sharply defined in the 

 older larva. 



Internally, sensory cristae ("crests") develop in each of 

 the ampullae of the semicircular canals, and patches of sen- 

 sory cells form the maculae ("spot") in the utriculus, sac- 

 culus, and lagena. Sensory membranes develop in the am- 

 phibian and basilar papillae. These membranes are formed 

 of sensory and supporting cells projecting halfway across 

 the lumen of the papilla. Each sensory cell has groups of 

 hairs forming a conical projection which is embedded in a 

 gelatinous and fibrous tectorial membrane. Fibers radiate 

 from the ends of the supporting cells into the tectorial mem- 



brane, and these interlace distally to form a support for the 

 margin of the membrane. 



A perilymphatic space forms in the mesenchyme between 

 the membranous labyrinth and the capsule wall, first in the 

 area of the fenestra vestibulae. From here it extends dorso- 

 posteriorly around the sacculus in contact with the amphib- 

 ian papilla, then around behind to the inner wall of the 

 otic capsule. It passes through the perilymphatic fenestra 

 and expands as a perilymphatic sac lying ventrolateral to 

 the medulla. 



Up to this point the development of the ear of the Bull- 

 frog is representative. The following description of the bron- 

 chial columella applies only to the Bullfrog (Figure 13-20 A) 

 and is important only as an example of the plasticity of this 

 area. A branch of the perilymphatic sac extends outward, 

 through the metotic foramen, in contact, above and inter- 

 nally, with the basilar papilla, and below with the dorsal 

 aorta. 



In the larval frog the contact with the dorsal aorta is re- 

 lated to a band of connective tissue which develops in the 

 dorsal aorta. This appears first on the medial wall and later 

 comes to span the aorta from top to bottom. This band, the 

 bronchial columella, is continued below the aorta to the top 

 surface of the lung. It acts in transmitting vibrations from 

 the lung through the aorta to the perilymphatic space and 

 directly up through this to the basilar papilla. In the course 

 of metamorphosis, the bronchial columella atrophies and is 

 lost. 



The loss of the bronchial columella in the Bullfrog is re- 

 lated to the development of the tympanic columella and 

 the middle-ear cavity, which forms from the first branchial 

 pouch. This cavity when formed is separated from the ex- 

 terior by a tympanic membrane; and it is connected with 

 the pharynx by the eustachian tube. The columella is formed 

 of the stapes and extracolumella; it spans the tympanic 

 cavity and conducts vibrations to the perilymphatic fluid at 

 the fenestra vestibuli. 



Xenopus, another anuran, is more typical of the tetrapod 

 (Figure 13-20 B). The perilymphatic canal passes between 

 the amphibian papilla, above, and the lagena, below. Be- 

 yond these it expands into a chamber which contacts the 

 basilar papilla. From here it constricts to pass through the 

 perilymphatic foramen to enter the metotic foramen and 

 reach the cranial cavity. 



In the salamander both the amphibian and basilar papil- 

 lae are usually developed, at least in the larva. The basilar 

 papilla is not found in neotenic forms such as Proteus and 

 Necturus. 



The endolymphatic duct enters the cranial cavity and 

 forms a midline endolymphatic sac in the salamander (Figure 

 13-21). This is pardy subdivided by septa and heavily pig- 

 mented in Necturus (Figure 13-6). In the frog, bilateral sacs 

 develop which extend forward and posteriorly. These join 

 and extend down through the spinal canal with a pouch 

 evaginating from the spinal column with each spinal nerve. 



406 • THE NERVOUS SYSTEM 



