Johnston, Forchrain Vesicle in Vertebrates. 533 



11. The telencephalon is bounded behind in yonng embryos by the 

 optic vesicles and primitive optic groove; in adults by the velum 

 transversum and the recessus postopticus; in mammals by a line or 

 plane passing immediately behind the -interventricular foramina 

 and the chiasma ridge. 



12. Externally this boundary is indicated by a furrow which in 

 mammals is very deep and has at its bottom the iissura chorioidea. 



13. The basal portion of this segment is reduced in volume owing 

 to the absence of motor nuclei and other structures. It is repre- 

 sented by decussating tracts (optic chiasma, commissures of Gudden 

 and Meynert) and perhaps by a certain amount of gray matter and 

 some longitudinal tracts. 



14. The dorsal portion of this segment is greatly enlarged and 

 increases in size and complexity in the vertebrate series. Its increase 

 is believed by the writer to be due to the development of the struc- 

 tures already present in this first brain segment in primitive verte- 

 brates. 



15. The sulcus limitans ends in the recessus prteopticus. All the 

 olfactory centers and the corpus striatum belong in the dorsal zone. 



16. The dorsal zone consists in other brain segments primitively 

 of visceral sensory and somatic sensory columns together with central 

 gray or correlating substance. The olfactory centers constitute the 

 visceral sensory portion of the telencephalon. The somatic portion 

 is represented in higher forms by the general cortex (neopallium), 

 in fishes possibly by the beginnings of this cortex and by the sensory 

 center for the nervus terminalis. The corpus striatum and epistri- 

 atum seem to contain the correlating material from which the 

 archipallium and neopallium have developed. 



17. The revision of the BJ^A terms made necessary by the new 

 facts brought out in this paper is indicated in the follo^ving table : 



Mesencephalon 



(Pars ventralis — ) pedunculus cerebri Bl^A. 



(Pars dorsalis — ) corpora quadrigemina BI*^A. 

 Diencephalon 



(Ventral and dorsal portions not clearly definable; four divisions 

 based on purely topographical features). 



