DEVELOPMENT OF THE NERVOUS SYSTEM 535 



The fore-brain 



As Herrick ('10) has pointed out, the cerebral hemispheres of 

 amphibia are divided into five parts which he terms olfactory bulb, 

 ventro-medial, ventro-lateral, dorso-lateral, and dorso-medial 

 parts. These divisions correspond to Gaupp's lobus olfactorius, 

 emenentia septalis, ganglion basale, formatio pallii lateralis and 

 emenentia pallii medialis. The two lateral parts are separated 

 by the sulcus Hmitans lateraHs (Gaupp) and the two medial parts 

 by the fissure limitans hippocampi (Herrick). 



In Desmognathus the relations are very much complicated, up 

 to and slightly after hatching, by the prominent flexure in the dien- 

 cephaHc region. Figures 35 and 36 show the medial and lateral 

 aspects of the brain of a larva just before hatching, or Stage XII. 

 Figure 42, the plane of which is shown by the dotted line a-a 

 (fig. 35), is a section through a brain of this age. The walls, which 

 later form the pars ventro-lateralis and dorso-lateralis, are very 

 massive, the medial walls being relatively thin, i.e., two to three 

 cells thick. The thickness of the lateral walls has reduced the 

 lateral ventricle and also the ventriculus impar so that they are 

 much smaller, relatively, than in the earlier stages. By examining 

 the series of models (figs. 27 to 33) a relative decrease in the size 

 of the ventricles of the fore-brain and an increase in the lateral 

 walls up to this stage will be noted. 



In a 13 mm. larva the cerebral flexure is much less pronounced 

 and consequent!}^ the relations simplified. Figure 43 is through 

 the caudal part of the olfactory bulb. There is no fusion of 

 olfactory bulbs in this or the later stages as in the frog. Figure 44 

 is just cephalad to the interventricular foramina. The membran- 

 ous septum is not nearly as extensive as it is at a later stage, when a 

 great increase in the length of the brain takes place. The ventro- 

 lateral part (striatum complex) is so massive that it nearly oblit- 

 erates the ventral part of the lateral ventricle. The dorso-medial 

 part, however, is less developed than in a later stage. The cells also 

 are arranged less densely here than in the other parts and approach 

 nearer to the surface. This will form the primordium hippocampi. 



JOIRNAL OF MORPHOLOGY, VOL. 25, NO. 3 



