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THE GERM LAYER DERIVATIVES 



Rhombocoel 



Mesocoe! 



Mesencephalon 



Epiphysis 



Neurocoel 



Notochord 



Tuberculunn posterius 



Diocoel 



Infundibulum 

 Optic chiasma 

 Optic recess 

 Torus transversus 

 Lamina terminahs 



Pharynx 



Hypophysis 

 Oral evagmation 



Early organogeny of the frog tadpole, showing the primary brain vesicles in 



sagittal section. 



imaginary line separates the anterior telencephalon from the more 

 posterior diencephalon of the forebrain. The telencephalon includes 

 the fused anterior neural folds which comprise the lamina terminalis. 

 This lamina will form a partition which will separate the paired 

 cerebral hemispheres by a longitudinal groove. 



The telencephalon is the embryonic cerebrum. Its cavity expands 

 laterally to give rise to the right (first) and left (second) lateral 

 ventricles and the surrounding thick-walled cerebral hemispheres, at 

 about the 12 mm. stage. These ventricles are laterally compressed. 

 In the frog the cerebral hemispheres are first differentiated at the 7 

 mm. stage but never become very large. The two telencephalic vesicles 

 are partially constricted off from each other but remain connected 

 by way of the tubular foramen of Monro, which opens into the 

 common (intermediate) third ventricle, known as the ventriculus 

 impar. The third ventricle overlaps and connects the telocoel and the 

 diocoel. The nervous tissue of the cerebral hemispheres is connected 

 transversely by the ventral anterior commissure, which is the original 

 torus transversus, and the anterior pallial commissure found postero- 

 mesially. The cerebral hemispheres become greatly thickened with 

 consequent enlargement and there is a parallel reduction in the size 

 of the contained ventricles. Antero-ventrally each grows out toward 

 the anteriorly placed olfactory placodes to establish connections 

 known as the olfactory lobes and nerves. The olfactory lobes arise as 

 a pair but subsequently become fused mesially. These are rather 



