The Brain of Diemydylus Viridescens 271 



Prosencephal. — The cerebrum is markedly imdeveloped, 

 especially in the region of the callosal eminence (compare 

 Fig. 35, 69 ; 36, 2 ; 37, 71 ; 16, 76). Owing to this the terma 

 and hence the portse have their cephalic boundary at a level 

 with the caudal portion of the olfactory lobes. The lack of 

 development of the callosal eminence is correlated with the 

 fact that barely a trace of the callosum is seen (Fig. 91), a 

 few fibers appearing on each side and even fewer crossing the 

 meson. The combined callosum and precommissure do not 

 rise much from the general level of the floor of the cavity 

 (Fig. 67). The precommissure is foreshadowed by the two 

 small white areas crossing the meson, while the striatum (Fig. 

 16, 78) shows one lamina of cinerea instead of two as in the 

 adult. 



Diencephal. — The chiasma and optic nerves are well de- 

 veloped as would be expected from the condition of the eyes. 



The infundibulum is much compressed cephalo-caudad and 

 no alba is developed in it (Fig. 81, 22). A saccus is not 

 formed, and the hypophj^sis is minute (Fig. 82). The roof of 

 the diencephal presents the same relations as in the adult, but 

 the opening between the supra- and postcommissures into the 

 epiphy.sis can be traced (Fig. 68). The 'epiphysis is relatively 

 larger than in the adult but its cavity is much depressed. 



The habenae are relatively very large (Fig. 4, 64), they ex- 

 tend far cephalad and partly overhang the paraphysis (Fig. 

 78, 91), and except for intervening pia they present a large 

 area upon the meson (Fig. 67). 



Plexuses.- The. paraphysis \s an almost straight tube with 

 an enlarged end (Fig. 67, 78, 79), surrounded by a few cells 

 and small vessels which constitute the rudimentary supraplex- 

 us, and have the same relation to the other plexuses as in the 

 adult (Fig. 66, 67). The dia- and auliplexuses have the same 

 relative position and extent as in the adult, but the endj'ma 

 with which they are covered is undifferentiated from other 

 endyma and the contained blood vessels are minute. The 

 paraplexuses arising from the auliplexus are small (Fig. 78), 

 and do not extend into the caudal part of the paracoeles as in 

 the adult (Fig. 21). 



