THE LATER DEVELOPMENT OF THE FROG 131 



growth of the wall, the paraphysis. That part of the prosen- 

 cephalon extending from the choroid plexus and epiphysis on 

 the dorsal side, to the inf undibulum on the ventral side, is known 

 as the diencephalon or between-brain, while the remaining ante- 

 rior portion is the telencephalon or secondary fore-brain. 



Considerably later (about 7 mm. or time of hatching) the 

 rudiments of the cerebral hemispheres appear, growing outward 



dw. 



hy. 



FIG. 41. Median sagittal section through the brain of an embryo R. fusca, 

 of 2.3 mm. length, but in a more advanced stage than that of Fig. 40. From Von 

 Kupffer (Hertwig's Handbuch, etc.). cd, Notochord; cw, rudiment of optic 

 chiasma; dw, dorsal thickening; e, rudiment of epiphysis; en, endodermal lining 

 of pharynx; hy, hypophysis; /, inf undibulum; It, lamina terminalis; M, mesen- 

 cephalon; P, prose ncephalon; R, rhombencephalon; sk, rudiment of olfactory 

 placode; tp, tuberculum posterius; tr, torus transversus. 



and forward from the sides of the telencephalon (Fig. 42, A). 

 These ultimately become very large and extend far in front of 

 the median region. They have thick inner and outer walls, and 

 contain extensions of the cavity of the telencephalon known as 

 the lateral or first and second ventricles, while the remaining 

 median cavity of the telencephalon and diencephalon is known 

 as the third ventricle. The ventricles of the cerebral hemi- 

 spheres are laterally compressed, and open out of the third 

 ventricle by a pair of openings known as the foramina of 



