ZOOLOGY: E. P. ALUS 
77 
definitive upper lip of the fish. In embryos of the Amniota, and 
unquestionably also in embryos of Heterodontus and those other 
Plagiostomi in which similar conditions may exist, the fold of the 
secondary upper lip is represented in the .maxillary and fronto-nasal 
processes, the fold of this lip having here been cut into two parts by 
its encounter with the nasal groove; and these two processes form the 
definitive upper lip. With the formation of these embryonic so-called 
processes the primary stomodaeum has been converted into a secondary 
one, and a portion of the external surface of the head is in process of 
being enclosed in the cavity of the mouth. In embryos of Ceratodus 
the supramaxillary fold is superadded to the maxillary process, and 
the stomodaeum becomes a tertiary one. 
In embryos of the Amniota the maxillary and fronto-nasal processes, 
representing the two parts of the fold of the secondary upper lip, lie 
primarily oral, respectively, to the lateral and mesial nasal processes 
as those processes are defined by Peter,^ the fold of the secondary upper 
lip thus passing across the oral edge of the nasal groove, as it does in 
the adult Chimaera, and not, as in the adult Heterodontus, in the line 
of the nasal processes and hence that of the future nasal bridge. The 
break between the two parts of the fold then forms the so-called naso- 
buccal groove, which is thus simply a partially closed portion of the 
future posterior nasal aperture and not a specially developed groove 
which cuts across the secondary lip and leads into the secondary cavity 
of the mouth. The two parts of the fold never, in the Amniota, com- 
pletely fuse with each other above the nasal groove, being always sepa- 
rated from each other by an epithelial line or membrane which is later 
broken through, and when a nasal bridge has been formed by the fusion 
of the nasal processes, the fold traverses that bridge. This bridge is 
thus the strict homologue of the bridge in the Teleostomi; for it is highly 
improbable that any new material has been brought to it by a simple 
fold of the dermis, as it is also improbable that such a fold, simply be- 
cause it passes across the bridge, can in any way change the morphologi- 
cal character of that bridge. That band of the external surface of the 
head that lies between the primary and secondary upper lips forms the 
primitive palate, and the nasal processes take part in its formation 
only as they are included, in part, in that surface. Where the nasal 
bridge lies wholly aboral to the secondary upper lip, as in the Teleostomi, 
it does not form a primitive palate and can in no way be compared with it. 
Aboral to the maxillary process of embryos of the Mammalia there 
is another process, or more properly a fold, which diverges slightly 
from the maxillary process and extends as far forward as the lachrymal 
