1897.] Cephalie Homologies. 941 
is an ectodermal invagination, which has a direct or acquired 
open communication with the entodermal cavity or archente- 
ron. Is there such an evagination in the Vertebrates? Un- 
questionably there is, and a large conspicuous and well known 
one, which gives rise to the olfactory pits and to the hypophy- 
sis proper. It is important to recall that the naso-hypophysal 
invagination is single, and we may homologize it with the 
Annelid ectodermal stomodaeum. It seems to me a very sig- 
nificant fact that in the lowest Vertebrates (the Myxinoids) the 
naso-hypophysal invagination opens through the entoderm 
into the pharynx. This connection, which has been known 
but the significance of which has remained obscure for three- 
quarters of a century, may, under the supposition we are con- 
sidering, be interpreted as the survival of the Annelidan con- 
dition. In other Vertebrates (Petromyzon and Gnathostomes) 
the opening mentioned does not recur, although contact be- 
tween the ectoderm of the invagination and the pharyngeal 
entoderm can, it is claimed, be observed in certain forms 
(Accipenser, etc). To avoid confusion, it must be pointed out 
that the hypophysis is only a part of the invagination, and 
neither it nor the general invagination are connected with the 
neuropore; the hypophysis of Vertebrates is, therefore, not 
homologous with the neuroporic pit, which Willey and others 
have erroneously named the hypophysis of Amphioxus and 
Tunicates. 
‘What is the actual vertebrate mouth? If we interpret the 
naso-hypophysal invagination as the homologue of the Anne- 
lidan mouth-gut, we find that the oral invagination 1s sepa- 
ted from the hypophysal by a fold, which becomes the upper 
jaw in Cyclostomes, and which is present in the embryos of 
all Vertebrates. (In Anmiote embryos this fold forms a ridge 
between Seesel’s pocket and the hyphophysis). It, therefore, | 
still remains necessary to look upon the vertebrate mouth as 
a new acquisition. It seems to me that, under the new con- 
ception of the head, Dohrn’s hypothesis is more promising 
than before. But beyond this I can add nothing to the ex- 
planations of the vertebrate mouth previously attempted. 
