172 VISION 



is similar to that of many other anamniotic vertebrates in consisting of 

 prominent periventricular cell groups and a more lateral sparsely celled neu- 

 ropil (Figure 2). In contrast, the thalamus of galeomorph sharks and batoids 

 (Figures 4, 5, 27) is characterized by marked thickening of the thalamic wall 

 and cellular migration away from the ventricle. The galeomorph thalamus not 

 only is larger than that of squalomorphs, but is also better differentiated into 

 distinct nuclei similar to the thalamus in birds and reptiles. 



Experimental studies on retinal projections in sharks (Ebbesson and Ramsey 

 1968, Graeber and Ebbesson 1972a, Northcutt 1976) and skates (Northcutt 

 and Boord, unpublished observations) reveal that the optic projections are 

 entirely crossed and that both dorsal and ventral thalamic nuclei receive optic 

 terminals (Figures 2-5). The decussating optic fibers form a lateral or marginal 

 optic tract that courses dorsally, terminating in the lateral neuropil of the 

 dorsal and ventral thalamus (Figures 2A, 4A). 



The rostral dorsal thalamus of Squalus consists of two major laminae: a thin 

 medial lamina and a larger densely packed lateral lamina with a sparsely celled 

 neuropil that receives retinal input (Figure 2A). Similar divisions can be 

 recognized in Mustelus, but there the thalamic wall is much thicker and the 

 retino-recipient zone contains as many neurons as the more medial thalamic 

 lamina (Figure 4A). 



More caudally, the lateral lamina of the dorsal thalamus is replaced by a new 

 neural population embedded in the optic tract and termed the superficial 

 pretectal nucleus (sp, Figures 2, 4, 20D, 27). Unlike the lateral lamina, the 

 rostral medial lamina of the dorsal thalamus continues caudally and expands 

 ventrally, assuming the shape of an inverted V (Figures 2B, 4B). Further 

 caudally, this lamina is finally replaced by nucleus interstitialis at midbrain 

 levels (Figure 5A). The medial lamina forms the bulk of the dorsal thalamus; it 

 is not a homogeneous plate, but can be divided into four or five distinct nuclei 

 based on cell size and packing densities. However, determination of the exact 

 number and the boundaries of these nuclei requires further experimental 

 information. 



Only two distinct dorsal thalamic nuclei (rostral thalamic and superficial 

 pretectal nuclei) receive visual input, but the ventral thalamus receives retinal 

 fibers throughout its entire lateral, rostrocaudal extent (Figures 2-5). Ros- 

 trally, the ventral thalamus also consists of two cell groups: a dorsal circular 

 nucleus and a ventral, more oblong nucleus with more densely packed cells 

 (Figure 2A, B). The ventrolateral edge of the ventral nucleus juts laterally 

 around the edge of the forebrain bundles (Figures 2A, 4 A) and may represent 

 an entopeduncular nucleus as in many other nonmammalian vertebrates. The 

 ventral thalamus extends far caudally beneath the optic tectum (Figures 2D, 

 4B), and as its caudal border is approached (Figure 20D) retinal fibers termi- 

 nate medially almost in contact with the periventricular cell bodies. More 

 caudally, the ventral thalamus is replaced by the midbrain tegmentum, which 

 is first recognized as a medial and dorsal continuation of the hypothalamus 

 (Figure 3A). 



The caudal dorsal thalamus or pretectum (Figures 2C, 4B, 20D) consists of 

 three cellular groups lying dorsal and lateral to the subcommissural organ and 



