90 THE ANCESTRY OF VERTEBRATES 



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, ^* hich 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 hypophysis cerebri, of which Stendell (191 4> 

 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 (J 905) 

 pictures of further advanced stages, gives rise to the rudiment 

 of the hypophysis, which moves under the cerebral vesicle 

 and applies itself to the infundibulum. Thus the hypophysis- 

 appears to be ot prostomial origin and is derived from the 

 piae-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 e.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 prostomiali 

 origin, just as the olfactory grooves. A possible relation* 



