221 
parallel to those which in Molluscs led to the strikingly 
similar Cephalopod eyes and in Annelids to the Alciopid 
eyes. We must conclude that the epichordal part of the 
medullary tube of Craniates, including the hind-brain, 
corresponds to the whole central nervous system of 
Amphioxus and to the stomodaeum in Annelids and that the 
praechordal forebrain, with the eyes, is a new acquisition 
of a quite different origin, but derived from the same source 
which in Annelids gives rise to the cerebral ganglia and 
the eyes. The inverted structure and encephalogenetic origin 
of the Craniate eyes are explained at once. Since the first 
publication of my theory, pricking experiments, to be de- 
scribed ín detail in the last chapter, have yielded a valuable 
confirmation of the views expressed above. Here may 
be mentioned the main results only. 
Situation of the animal pole. — While in the first publica- 
tion of this theory (1913) 1 assumed that the praechordal 
part of the brain was to be derived from the infolding of 
the whole apical plate or episphere of the worm larva. [ after- 
wards (1916) concluded that this conception could not be 
correct, for, besides the fore-brain, the episphere must furnish 
also in' Craniates, just as in Annelids and in Amphioxus, the 
ectodermal wall of the snout. Thus the fore brain can be 
derived only from part of the apical plate or of the ectodermal 
wall of the prostomium. This must be the part contiguous 
to the mouth, i.e. the neuropore of Amphioxus. Now 
in worms and molluscs we find in the centre of the apical 
plate of the larva the animal pole of the egg, being the aboral 
pole of the gastrula, indicated as a rule by the presence 
of the polar bodies and by the regular radiate arrangement 
of the cleavage cells round it. If the whole apical plate 
were to be transformed into the fore-brain of Craniates 
we should expect to find the animal pole in the latter on 
the cerebral plate, and this conclusion was drawn in my 
first article. If, however, as 1 have corrected it afterwards, 
only half the apical plate gives rise to the fore-brain of 
Craniates and the other half to the ectodermal investment 
of the prostomium, we may expect to find the animal pole 
in the vicinity of the anterior border of the cerebral plate, 
known as the transverse cerebral fold, or of the neuropore 
after the closure of the brain vesicle. Pricking experiments 
on amphibian and fish eggs, to be described in the last 
Chapter, fully confirm this conclusion. 
