236 
of the prostomium incorporated into the brain, i.e. from the 
ventral half in Annelids, while the ciliated pits belong to the 
dorsal half and are not closed round. Thus we can only 
assume, that the connection between the brain and the 
olfactory pits in Vertebrates is a secondary one compared 
to that in Annelids. 
Primary brain-axis.— A number of authors have discussed 
the question of the primary brain axis and its anterior end 
and have come to divergent conclusions. This brain axis 
of course is the forward continuation of the axis of the 
meduliary tube, the anterior dilated part of which is con- 
sidered as being represented by the brain. According to 
Vv. BAER (1828), MIHALKOVICS (1877), HIS (1893, p. 98), 
KOLTZOFF (1902, p. 553) and HATSCHEK (1892, p. 139, 1909, 
p. 497) the original fore-end of the Craniate brain, corres- 
ponding to the neuropore of Amphioxus, is to be found in 
the infundibulum, and the part that lies in front of the 
latter, the praechordal part has secondarily grown out over 
it by the strong development of the originally dorsal parts. 
This conception finds its most striking expression in VAN 
WYHE's (1882, p. 39) view that the nervus olfactorius in 
reality is the second, the nervus opticus the first nerve of 
the head. V. KUPFFER (1894, 1926), on the contrary, starts 
from the idea, that, to find the fore-end of the brain-axis 
we must look for the neuropore of Craniates, which in his 
opinion is directly comparable with the neuropore of Amphio- 
xus (cf p. 224). He finds the latter in the sturgeon as a point 
where the brainwall remains fused with the epiderm of the 
head for a somewhat longer time, the angulus terminalis (HIS) 
or recessus neuroporicus (cf. fig. 13), situated more dorsally, 
just in front of the lamina terminalis or front-wall of the brain. 
It indicates at the same time the anterior end of the medullary 
Suture, just as in Amphioxus, and the front-wail of the 
brain has been formed by the growing up ofthe transverse 
brain fold of the cerebral plate. To this conception HIS 
objects, that, in transverse sections, the medullary suture may 
be shown to continue in front of the angulus terminalis as 
a frontal suture, probably reaching to a point where in 
other forms, e.g. in Petromyzon, a much more terminally 
situated neuropore, or better: an indication of the latter, 
is found. According to HIS and HATSCHEK both the 
“angular” and the “terminal’ neuropore represent the 
extremities of an original slit-like neuropore, as observed by 
