224 
into two regions is very evident. Immediately after the 
closure of the medullary plate we can distinguish, with 
V. KUPFFER (1905), the archencephalon and the deuteren- 
cephalon, the former praechordal and giving rise to the 
optic vesicles, and the latter epichordal. The two are 
separated from each other by the plica ventralis, a transverse 
fold of the ventral wall of the brain, right over the fore-end 
of the notochord. The archencephalon is thus a well defined 
vesicle; the deuterencephalon tapers insensibly, without a 
distinct limit, into the medullary tube of which itis indeed 
only the anterior dilated part. The plica ventralis indicates 
at the same time the place of the cephalic flexure and lies 
right over the sella turcica which, after GEGENBAUR, (1872, 
119), indicates the limit between the segmented “verte- 
bral” and the unsegmented “praevertebral” part of the skull. 
From the archencephalon the telencephalon and the dience- 
phalon afterwards develop, from the deuterencephalon, whose 
cavity becomes the fourth ventricle, the myelencephalon 
and, if present, the metencephalon. For the mesencephalon, 
whose cavity becomes the iter, it is hard to say from which 
of the two it is to be derived and if it is to be derived 
from either of them. It lies exactly above the plica ventralis. 
At any rate it is in the region of the isthmus, between meso- 
and metencephalon, that we have to look in Craniates for 
the neuropore of Amphioxus which represents the old mouth. 
From the above considerations it follows that the brain vesicle 
of Amphioxus is to be compared with the deuterencephalon 
of Craniates and not with the archencephalon, as KUPFFER 
himself supposed. From this point of view it might appear 
adequate to interchange the names arch- and deuterence- 
phalon, the latter phylogenetically being present before the 
archencepalon. 1 will not, however, propose a change of 
names when introduced by such an authority as V. KUPFFER 
and generally adopted. Not only would such a change 
give rise to confusion but arguments may be equally 
well adduced for the present nomenclature, for the archen- 
cephalon, together with the eyes, develops from the same 
source as the cerebral ganglia and eyes in worms, molluscs 
and arthropods, viz: from the apical plate, and that their 
homologue is absent in Amphioxus is evidently due only 
to secondary circumstances. In this respect then the archen- 
cephalon has older claims to the name of brain than the 
deuterencephalon. 
