The Brain of Diemydylus Viridescens 277. 



ture and growing spaces of this period. With continued 

 growth the structures become more compact. 



The growth of the mesal walls of the cerebrum is slow, 

 the cells and alba forming the callosal eminence being gradu- 

 all}' added until, before the adult stage is reached, it is fully 

 formed. The callosum and precommissure continue to be sep- 

 arated by cinerea up to adult life, and even in some small 

 adults the condition persists. 



The compressed form of the infiindibuhirn is lost by the 

 middle red stage. The saccus is sometimes quite convoluted 

 by the end of terrestrial life but this is apparently a variable 

 condition. The hypophysis in the late red forms shows the 

 tubules widely open, but the large relative size of the organ 

 is not attained until adult life, though in the latter the tub- 

 ules again are more compressed. The original close approxi- 

 mation of the notochord to the hypophysis (Fig. 67) is lost by 

 the middle red stage. The paraphysis is nearly as convo- 

 luted in the small red forms as in the adult owing to the ex- 

 tensive development of blood vessels in the supraplexus 

 which takes place at this stage. 



By the end of aquatic larval life the roof of the mecencephal 

 has coalesced, the broad dorsal band of cinerea remains 

 through this stage, and fully grown red forms show a line of 

 cinerea on the dorsal side where the union took place. (See 

 p. 293). In the middle red stage the layers of cinerea are 

 forming while in late red forms the layers are more clearly 

 separated than in the adult — especially is the endyma separate 

 from the next cellular layer. Traces of the caudal expansion 

 of the mesocoele remain until late red stages (Fig. 8). 



The cerebellum is as large proportionally by the middle red 

 stage as in the adult, and at that time the extreme latero- 

 cephalic projection of the lateral recesses of the epiccele has 

 disappeared. 



The metaplexus shows a median fold in a 16 mm. larva 

 while a mere trace of the metapore has appeared and consists 

 in the abrogation of a few cells of endyma (Fig. 57), In the 

 large aquatic larvae the metapore has not increased in size but 

 the metaplexus is as complex as in the adult. By the middle 



