"DRTGIfrSND^STRrCTURE" OT THE HEAD — gj- 



of the hypophysis to the animal pole would no doubt be- 

 worth of a closer investigation. Hatschek (1909, p. 511), 

 in Petromyzon, comes to the conclusion: "Die Hypophy- 

 senecke liegt unmittelbar an dem (oder "bezeichnet den") 

 primitiven vorderen Pol des Craniotenkorpers. Dieser 

 morphologisch ungemein wichtige Fundamentalsatz, betreffend 

 den vorderen Korperpol der Cranioten ist aus dem Verhalten 

 des Petromyzontenembryo ohne weiteres abzuleiten". I do not, 

 however, think it probable that the animal pole itself is the 

 point from which the hypophysis originates, since the latter 

 appears to arise always behind the olfactory grooves, not 

 in front of them as might be expected in this case. It seems 

 evident, however, that it is a prostomial organ. 



In Selachians, Teleosteans, Sauropsida and Mammals, 

 hqwever, we see the hypophysis originate from the roof of 

 the mouth involution, in the same way as, in higher Verte- 

 brates, the olfactory organ also acquires relations to the 

 palate. This makes us infer that the praecerebral region of 

 the apical plate, or part of it, has moved into the mouth, 

 investing the palate. One would be inclined to connect this 

 curious phenomenon with the circumstance that the mouth 

 itself in higher Vertebrates has moved forward and has 

 acquired a more terminal position, but this is already the 

 case in Amphibians, while in the Elasmobranchs, with their 

 ventral mouth, the hypophysis originates from the mouth 

 involution. It is also possible, that we have to do here 

 only with the displacement of a differentiation and not with 

 that of the cells themselves wh»ch produce the rudiment 

 of the pituitary body. However this may be, the relations 

 of the palate with such prostomial organs as the hypophysis 

 and the olfactory grooves may be adduced as a valuable 

 argument in favour of the view that the secondar}' mouth 

 of Craniates has broken through, not between the first and 

 the second segment, but at the limit of the prostomium and 

 the first segment, and that accordingly the latter is repre- 

 sented by the mandibular segment. 1 shall further indicate the 

 hyoid segment as the second, the glossopharyngeus segment 

 as the third (after Van Wyhe the fifth), the segment of 

 the primary vagus as the fourth (VAN WYHE's sixth), etc. 



Derivation of mouth from gill-slits. — It can not be said 

 that DOHRN's (1875) hypothesis of the derivation of the 

 mouth of Craniates from two fused gill-slits is supported 

 buite convincingly by ontogeny. DOHRN (1881) himself 



