MORPHOLOGY OF THE FOREBRAIN 483 



THE PARATERMINAL BODY 



Elliot Smith has applied the term precommissural body, or 

 paraterminal body, to certain structures associated with the 

 lamina terminalis and septum in the median wall of the hemi- 

 sphere; and in his penetrating analysis of the relation of the hip- 

 pocampus to the remainder of the cerebral hemisphere ('03, 

 p. 489) he emphasizes the intimate relation of the hippocampus to 

 the paraterminal body and adds, "a stud} 7 of the relations of tne 

 primordium hippocampi to the paraterminal body in the Ichthy- 

 opsida lends support to the view that the hippocampus may be 

 merely the specialized upper part of the primitive paraterminal 

 body." 



In testing the validity of this hypothesis, it at once appeared 

 that the diponan and reptilian paraterminal body, as defined by 

 Elliot Smith, is a two-fold structure, whose components are prob- 

 ably distinct in origin and subsequent evolution. 



(1) The ventral component of the paraterminal body lies with- 

 in the pars ventro-medialis of the hemisphere and is a basal olfac- 

 tory center, developed originally as a terminal nucleus of the ven- 

 tral division of the tractus olfactorius medialis, to which have been 

 added olfactory fibers of the third order, ascending fibers from the 

 hypothalamus and other connections. Within it is differentiated 

 the nucleus medianus septi, some of whose cells form the "bed" 

 of the commissures of the lamina terminalis. Others of these 

 cells grow upward along the course of the fimbria to form the pars 

 fimbrialis septi (Kappers), while still others are related to the 

 fibers of the commissura pallii posterior in certain reptiles. This 

 component is always morphologically ventro-medial. I rec- 

 comn end that the use of the term precommissural body be limited 

 to this component. 



(2) The dorsal component of the paraterminal body, as de- 

 scribed by Elliot Smith for Lepidosiren, belongs in the dorso- 

 median wall of the hemisphere and in some urodeles (particularly 

 larvae) the corresponding structure is almost completely separated 

 from the ventral component by the membranous septum ependy- 

 male. It is, in fact, the primordium hippocampi. The hippo- 



