MORPHOLOGY OF THE FOREBRAIN 453 



dium hippocampi, which, as will appear immediately, I exclude 

 from it. The nuclei of the hippocampal commissure and tractus 

 cortico-habenularis (see p. 461) should also be excluded. 



The most highly differentiated cortex of reptiles is the dorso- 

 medial. Its relations to the ventro-medial part of the hemisphere 

 have been investigated in adult brains of several species of rep- 

 tiles and in the extensive series of embryos in the Harvard Embry- 

 ological Collection. 



Chelonia. We shall consider first the turtles. In transverse 

 sections of an embryo of Chrysemys marginata of 16.7 mm. 

 (Harvard Embryological Collection, no. 1092) the ventro-medial 

 part, or septum, has already attained great size; and, although 

 the process of proliferation of neuroblasts from the central grey 

 is still active, the principal regions of the adult hemisphere can 

 be recognized. The dorso-medial cortex is well defined; the lateral 

 cortex less clearly. A section through the middle of the septum 

 (fig. 43) shows that the medial wall of the hemisphere is subdi- 

 vided, as in Amphibia, by a definite zona limitans into dorsal 

 (hippocampal) and ventral (septal) parts; but the lateral wall 

 does not conform to the amphibian type. The tuberculum ol- 

 factorium, precommissural body (nucleus medianus septi) and 

 nucleus accumbens septi can be recognized. The locus of the 

 nucleus lateralis septi lies between the two last. Fibers of the 

 fornix and commissura hippocampi systems are seen crossing the 

 zona limitans. 4 section taken 150 micra farther caudad (fig. 

 44) shows dorsal to the zona limitans a thin portion of the wall 

 which contains fimbria fibers and a few nuclei. In a section 

 taken 50 micra farther back and immediately in front of the inter- 

 ventricular foramen (fig. 45) an extensive septum ependymale 

 appears and above it a slight thickening containing the fimbria 

 fibers, the limbus medullaris of His. This belongs to the dorso- 

 median part of the hemisphere and represents a small residue of 

 the unspecialized amphibian primordium hippocampi, the re- 

 mainder of the primordium having been transformed into cortex 

 hippocampi (dorso-medial cortex). The commissura hippocampi 

 crosses in tne lamina terminalis in the plane of this section, its 

 fibers having curved downward rostral to the interventricular 

 foramina and septum ependymale. 



