424 C. JUDSON HERRICK 



blystoma resembles closely a corresponding section of the frog, 

 save for the absence hereof a clearly defined zona limitans lateralis 

 (fig. 12). The pars ventro-medialis is thickened by the enlarged 

 nucleus medianus septi. 



As we approach the foramen interventriculare the medial wall 

 below the primordium hippocampi, containing the nucleus me- 

 dianus septi, becomes thinner (fig. 13) in the site of the larval 

 septum ependymale, and for about 100 micra immediately ros- 

 tral to the foramen the larval septum ependymale is preserved 

 (fig. 14). No cells of the nucleus medianus extend caudad above 

 the foramen. 



The wide larval septum ependymale is almost obliterated by the 

 growth into it from below and from in front of cells of the nucleus 

 medianus septi. In larvae of 35 mm. this movement is in proc- 

 ess, as shown by fig. 24, which illustrates a cross-section taken in 

 a plane corresponding to fig. 13 of the adult. 



At the level of the interventricular foramen the pars ventro- 

 medialis is reduced in size and no clearly defined nucleus medi- 

 anus septi is here present (fig. 15) in either larvae or adults. The 

 pars dorso-medialis (primordium hippocampi) is in this region 

 large and well differentiated in the adult, but in young larvae it 

 is smaller than at its rostral and caudal ends. Compare fig. 15 

 (adult) with fig. 4, a section of the 17 mm. larva taken from the 

 same part of the hemisphere. A corresponding section from a 

 larva of 35 mm. is substantially the same as that of the 17 mm. 

 specimen. The ontogeny shows, in fact, that the primordium 

 hippocampi develops wholly within the dorso-median wall of the 

 hemisphere and that its histological differentiation begins at its 

 rostral and caudal ends, the middle part becoming differentiated 

 at a later stage. The morphological significance of this fact will 

 be commented upon in the discussion on page 487. The compo- 

 sition of the fimbria complex is as already described for the 17 

 mm. larva (p. 418). It receives secondary olfactory fibers from 

 the tractus olfactorius dorso-medianus. The medullated columna 

 fornicis fibers are confined to its rostral end. The unmedullated 

 fibers passing between the primordium hippocampi and the nu- 

 cleus medianus septi connect with all parts of the primordium in 



