orb 
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 Craniotenkörpers. Dieser 
morphologisch ungemein wichtige Fundamentalsatz, betreffend: 
den vorderen Körperpol der Cranioten ist aus dem Verhalten 
des Petromyzontenembryo ohne weiteres abzuleiten”. 1 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, 
however, 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 teiminal 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 which 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 secondary 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. l 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. 
