464 C. JUDSON HERRICK 



The subcortical centers in the median wall of the reptilian hemi- 

 sphere can be more precisely analyzed. The "septum" of earlier 

 authors (intraventricular lobe of C. L. Herrick, paraterminal 

 body of Elliot Smith, area precommissuralis septi of Kappers, 

 eminentia medialis hemisphaerii of Kupffer) is of two-fold origin : 



(1) The larger antero- ventral component is a derivative of the 

 primitive ventro-median area. The differentiated cellular mass 

 in the dorso-caudal part of this ventral component is comparable 

 with the nucleus medianus septi of the Amphibia, and to this 

 mass I have applied Elliot Smith's name precommissural body. 



(2) The small remainder of the "septum" is a vestige of the un- 

 specialized primordium hippocampi of the Amphibia. It is of 

 dorso-medial origin and is morphologically of quite different 

 type from the precommissural body. 



The fissura arcuata has given rise to much controversy in mam- 

 malian neurology and in reptiles its morphology is equally diffi- 

 cult. In mammals the fissura hippocampi is a total fold of the 

 median wall of the hemisphere and His has clearly shown ('04) 

 that the fissura arcuata of the foetal brains which he has described 

 is the precursor of this fissure. In mammals the hippocampus, 

 or cornu Ammonis, is the part of the cortex which is folded into the 

 ventricle by this fissure. In embryonic and adult reptiles (figs. 

 47, 54, 60, 66, 67) we have a similar fold of the median wall of the 

 hemisphere which is commonly called fissura arcuata. The cor- 

 tex hippocampi sometimes extends downward into the dorsal 

 lip of this fissure, but never in reptiles into the ventral lip which 

 is always formed by an unspecialized vestige of the primordium 

 hippocampi containing fimbria fibers and sparse cells. Since the 

 reptilian cortex hippocampi above this fissure was unquestionably 

 derived from the dorsal part of the amphibian primordium hip- 

 pocampi and since chat portion of the reptilian primordium hip- 

 pocampi which forms the ventral wall of this fissure is in like man- 

 ner destined in mammals to be transformed into cortex hippo- 

 campi, I think it is clear that the fissura arcuata of the lacertilian 

 embryo occupies the same relative position in the wall of the cere- 

 bral hemisphere as that of the mammalian embryo. Of course 

 it does not necessarily follow that the fissures are homologous, 



