533 
is that this sulcus is not present in embryonic brains and even in 
specimens of Acipenser 30 cm in length it is a very broad, shallow 
groove. The formation of a deep external sulcus is a result of the 
Fig. 3. Transverse section of the forebrain of Ameiurus of 19 mm. p.c.b. pre- 
commissural body. r.p. recessus praeopticus. s.a. somatic area. Zr.p. tractus pallii. 
t.f. taenia fornieis. From JOHNSTON 1911 b. 
Fig. 4. Transverse section of the forebrain of adult Amia. b.b. basal bundle. 
c.hipp. commissure of the primordium hippocampi. s.e. suleus externus. tr.olf.c.h. 
crossed bundle of the medial olfactory tract to the primordium hippocampi. 
hypertrophy of the gray matter lining the ventricle and of the ever- 
sion of the brain wall. This external sulcus is therefore less funda- 
mental than the internal sulci or the zdna limitans. It should be 
