ELASMOBRANCH BRAIN ORGANIZATION 167 



dorsally into the central and superficial zones (Leghissa 1962), contacting 

 optic terminals of the superficial zone. The proximal dendritic shafts of the 

 pyriform cells pass through the terminal zones of telencephalic, tectal com- 

 missural, and ascending spinal afferents, with which they probably form 

 synapses. The axons of these cells arise from the soma, or a dendritic shaft, 

 and enter the tectal efferent tract (layer 3) or the intertectal commissure, or 

 else ramify within the central zone. All studies to date (Table 5) have 

 recognized the same periventricular layers, though Houser (1901) excluded 

 these layers (his central gray matter) from the tectum proper, as has tradi- 

 tionally been done in describing the mammalian optic tectum. 



The central tectal zone consists of a deep fiber layer (layer 3) and a 

 superficial layer of fibers and scattered neurons (layer 4). The neurons of 

 layer 4 are termed large multipolar cells by Leghissa (1962) and send their 

 dendrites into the superficial zone and their axons deep into layer 3. 



Layer 3 is composed of axons that form the descending tectal efferents 

 and intertectal commissural fibers (Ebbesson 1972). This layer also contains 

 ascending fibers carrying sensory information into the tectum (Hayle 1973a). 

 The tectum also projects rostrally to the pretectum and thalamus, terminating 

 in nuclei that also receive retinal inputs and terminating caudally in the 

 ipsilateral and contralateral medullar reticular formations (Ebbesson 1972). 

 In other vertebrates with migrated tectal neurons, the tectal neurons that 

 project rostrally are located in the superficial and central zones, while the 

 cells that give rise to the descending tracts originate predominantly in the 

 periventricular zone. Similar experiments have not been performed for 

 cartilaginous fishes, but it is likely that they possess a similar organization. 



It is evident from Table 5 that differences in interpreting the organization 

 of the upper half of the tectum account for the variation in tectal nomencla- 

 ture. It is difficult to establish the exact dorsal boundary of the central zone 

 and the number of layers in the superficial zone. This difficulty stems from 

 differences in interpreting the course of incoming optic fibers and from 

 actual differences in development of layer 5 in sharks. Until recently, the 

 optic fibers were believed to enter the tectum in layer 6, then turn ventrally 

 to terminate in layers 4 and 5 (Kappers et al. 1936). However, experimental 

 studies (Schroeder and Ebbesson 1975, Northcutt 1976) demonstrate that 

 the optic fibers enter more deeply (layer 5 and upper part of layer 4) and 

 turn dorsally to terminate predominantly, if not solely, in layers 5 and 6. 



The pattern of optic terminals in the tectum appears different in squalo- 

 morph and galeomorph sharks (Figures 2-5). In squalomorphs, both optic 

 fibers of passage and their terminals are located above the dense tectal cell 

 layer; in galeomorphs, such as Mustelus (Figures 4, 5), Ginglymostoma, 

 Galeocerdo, and Negaprion (Ebbesson and Ramsey 1968, Graeber and 

 Ebbesson 1972a), the fibers of passage course in layers 4 and 5 and terminate 

 in layers 5 and 6. 



Present data are insufficient to determine whether the dense cell layer in 

 Squalus is homologous to layer 4 or layer 5 in Mustelus. It is possible that 

 both layers exist in Squalus, and that layer 5 is not as well developed as in 



