Functional factors in the iiiorpliolDj^y of the forehrain of fishcs 177 



its telencephalic connections are the primordia of the lobus pyriformis 

 and amygdala. The lateral forebrain bundle also contains ascending so- 

 matic sensory projection fibers (tractus thalamo-frontalis) from the pars 

 dorsalis thalami which distribute widely throughout the lateral wall of the 

 hemisphere and not to a definitely circumscribed área somática as in some 

 fishes. The ventro-lateral part of the hemisphere wall is an undifferen- 

 tiated olfacto-striatal primordium from which there descends in the late- 

 ral forebrain bundle a strong tract of fibers distributed to the ventral part 

 of the thalamus, dorsal part of the hypothalamus, and pedunculus cere- 

 bri. These fibers are comparable in a general way with those of fishes 

 termed tractus strio-thalamicus ¡n this paper. 



If, now, with these amphibian connections in mind we review the di- 

 telencephalic connections of fishes, the amphibian type is found to be 

 foreshadowed, but with bewildering diversity of detail, in the various spe- 

 cies. The stria medullaris complex and the medial forebrain bundle are 

 always recognizable, but the elements of the lateral forebrain bundle are 

 often dispersed or combined in different patterns. In no fishes which 

 have been described does the composition of the lateral bundles conform 

 closely to the amphibian type. The entire área olfactoria dorsalis is 

 connected with the hypothalamus by the massive tractus pallii 1 Johnston), 

 a lateral tract which is probably the precursor of the olfactory projection 

 tract. The área olfactoria lateralis is connected by descending fibers 

 with the hypothalamus; these probably are comparable with the tractus 

 olfacto-hypothalamicus lateralis and tractus strio-thalamicus in the lateral 

 forebrain bundles of Amphibia. The primordial striatum of fishes farea 

 olfacto-somatica) is not completely differentiated away from the área ol- 

 factoria lateralis in teleosts. Apparently it approaches this condition 

 most closely in elasmobranchs. In teleosts, as in urodeles, its neurons 

 send their axons downward into both the ventral thalamus and the hypo- 

 thalamus (tractus strio-thalamicus). Associated with these fibers in the 

 carp, according to Sheldon, are ascending somatic sensory fibers from 

 the thalamus comparable with the thalamo-frontal tracts of Amphibia. 

 In all fishes there is behind the área olfacto-somatica and área olfactoria 

 lateralis a differentiated área somática (Johnston) which has its indepen- 

 dent system of ascending and descending thalamic projection fibers and 

 which, in turn, is connected in Acanthias with the área olfactoria latera- 

 lis by a correlation tract. 



