EMBRYOLOGY OF TADPOLE AND CHICK 



571 



e. 



hm. 



level of the olfactory recess in the anterior wall of the fore-brain. It 

 extends ventrally to where the optic stalks protrude. From the drawing 

 (Fig. 333) it will be noticed that a thickening occurs in the region of the 

 attachment of the optic stalks to form the torus transversus (Fig. 333, tr) 

 and the beginnings of the optic chiasma (Fig. 333, cw) and thalami. 

 The torsus transversus becomes the seat of the anterior commissure 

 (Fig. 333, cpa) as well as other commissures of the brain. 



The narrow depression between the thickenings just mentioned 

 forms the recessus opticus (Fig. 333, ro), which is the passage to the 

 cavities in the optic stalks. 



The infundibulum (Fig. 333, J) is an outgrowth of the posterior 



portion of the fore- 

 brain where it extends 

 backward under the tip 

 of the notochord. 



The epiphysis or 

 pineal body (Fig. 

 333, e) is an evagina- 

 tion from the dorsal 

 wall of the fore-brain 

 at its posterior limit 

 where the wall has be- 

 come quite thin. 



The choroid plexus 

 (Fig. 333, pch) is the 

 non-nervous portion of 

 the roof of the fore- 

 brain which has be- 

 come thinned out con- 

 siderably, and in which 



the blood vessels lie. 

 This whole region is 

 pushed into the third 

 ventricle. 



The habenular 

 ganglia and the haben- 

 ular commissure (Fig. 

 333, ch) develop be- 

 tween the choroid 

 plexus and the 

 epiphysis. 



The paraphysis (a 

 dorsal growth) devel- 

 ops in front of the habenular ganglia and commmissure considerably 

 later. 



Fig. 333. 



Median sagittal sections through the brain of the frog. A. 

 Of a larva of R. fusca of 7 mm. in which the mouth was open*. 

 B. R. esculenta at the end of metamorphosis, c, Cerebellum ; co, 

 anterior commissure ; cd, notochord ; ch, habentflar commissure ; 

 cp, posterior commissure ; cpa, anterior pallial commissure ; cq, 

 posterior corpus quadrigeminum ; ct, tubercular commissure ; cw, 

 optic chiasma ; d, diencephalon ; dt, tract of IV cranial nerve ; 

 e, epiphysis ; hm, cerebral hemisphere ; Ay, hypophysis ; J, ijn- 

 f undibulum ; M, mesencephalon ; Ml, myelencephalon ; Aft, 

 metencephalon ; p, antero-dorsal extension of diencephalon ; pch, 

 choroid plexus of third ventricle ; R, rhombencephalon ; rm, 

 recessus mammillaris ; ro, optic recess ; se, roof of diencephalon ; 

 t, telencephalon ; tp, tuberculum posterius ; tr, torus trans^versus 

 (telencephali) ; vc, valvula cerebelli ; vi, ventriculus impar 

 (telencephali) (third ventricle"). (From Von Kupffer, Hert- 

 wig's Handbuch, etc.) 



