ORIGIN AND STRUCTURE OF THE HEAD 35 



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 in 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) I 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. I 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 I 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. 



