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the ectoderm of the prostomium would have invaginated’ 
into the mouth to invest the palate. This circumstance- 
seems to me especially to plead for the view that it is- 
between the prostomium and the first segment, which in 
this case would be the mandibular segment, that the mouth. 
in Craniates has broken through. One of the facts that 
may be adduced in favour of this view is the mode of 
origin of the hypop/.ysis cerebri, of which STENDELL (1914) 
some years ago gave a short review. 
Relation of hypophysis and olfactory pits to the mouth — 
In Rana esculenta the ectoderm not only of the medullary 
and the. cerebral plate but also in front of the latter and 
of the animal pole i. e. in the region which represents. 
the extra-cerebral part of the apical plate, shows a con- 
siderable thickening and a distinct demarcation of a thin 
superficial and a thick basal layer (fig:. 1 and 2 of the plate). 
While now in the cerebral and medullary plate, as observed 
by ASSHETON (1909), this demarcation soon becomes less 
evident, the superficial layer fusing with the basal one and, 
after the folding in of the plate, evidently giving rise to- 
the ependyme of the neural tube, an entirely different procesS- 
takes place in the prae-cerebral part of the apical plate. 
Here the demarcation of the superficial and the basal layer 
becomes more and moie evident. the basal layer detaches- 
itself more and more from the superficial one which now 
alone acts as ectoderm, overgrowing the closing cerebral 
folds (figs. 3 and 4 of the plate). The detached basal layer 
forms a lump of cells which, according to V. KUPFFER's ()905) 
pictures of further advarced stages, givesriseto the rudiment 
of the hypophysis, which moves under the cerebral vesicle 
and applies itself to the infundibulum. Thus the hypophysis- 
appears tc be of prostomial origin and is derived from the 
pie-cerebral region of the apical plate, which also gives 
rise to the olfactory grooves. In other groups of Ichthyopsids 
the same holds true, sometimes the first rudiment is not 
solid, as in Amphibians, but has the form of an invagination 
of the ectoderm just in front of the neuropore, as, judging 
from V. KUPFFER's (1905) figures, seems to be the case €.g. 
in Acipenser sturio. This is particularly obvious in the well- 
known case of Petromyzon, where the olfactory groove takes- 
its origin from the same invagination. Thus it is evident 
that in lower Vertebrates the hypophysis is of prostomial 
origin, just as the olfactory grooves. A possible relation 
