494 Journal of Comparative Neurology and Psychology. 



The only important difference between Amblystoma and Squalus is 

 that anterior head cavities are not formed in Amblystoma. In later 

 stages the preoral entoderm and median mass proliferate as mesen- 

 chyme, so that essentially the same end is reached as in Squalus. 

 In some embryos are found indications of a connection of the 

 archentoderm with the hypophysis through the preoral entoderm, 

 but this and the details of the formation of the hypophysis will be 

 described in another paper. 



As development proceeds the same shifting in the relations of the 

 optic vesicles is seen as has been described in Squalus. In the lateral 

 wall of the forebrain vesicle a thickening is formed which runs 



Fig. 31. 



Fig. 32. 



Fig. 31. Amblystoma p., nearly the same stage as that shown in Fig. 30, 

 median sagittal section of forebrain and midbrain, x 40. 



Fig. 32. Amblystoma p., later stage, median sagittal section. Note the ex- 

 treme curvature of the brain in this and following stages, x 40. 



from the terminal ridge in the middle line obliquely latero-caudad 

 across the primitive optic groove. This thickening is formed in 

 anticipation of the ingrowth of optic tract fibers and may be called 

 the optic ridge. It separates the optic vesicles from the primitive 

 optic groove and causes them to be connected by the terminal pit. 

 Figs. 25, 28, 29, 30 show this in sections and models. 



By the time the optic ridge is formed the floor of the forebrain 

 vesicle has become depressed to form broad primitive inferior lobes 

 and in the caudal wall of this a mammillary recess marks the begin- 

 ning of the mammillary bodies (Figs. 26 and 27). This is bounded 



