LIPS AND NASAL APERTURES IN FISHES 187 
nasal apertures. When, in the adults of any of these fishes, the 
secondary upper lips meet and coalesce at the symphysis, the 
maxillary processes also meet there and coalesce, and that band 
of the external surface of the head which, in earlier embryos, 
lies between their anterior ends is enclosed in the buccal cavity 
as a part of the secondary addition to that cavity. If the 
anterior ends of the maxillary process were to approach each 
other closely, but not fully to meet and coalesce, a median 
incisure would be left in the definitive upper lip. 
In embryos of all of the Amniota the fold of the secondary 
upper lip has been cut into maxillary and fronto-nasal portions 
by its passage across the nasal groove, as it is in certain fishes, 
and as each of these two portions of the lip, or so-called proc- 
esses, lies at first oral to the corresponding nasal process, as 
those processes are defined by Peter (’06), the fold of the 
secondary upper lip must here have passed across the oral nasal 
aperture and not between the two apertures. In Mammalian 
embryos the fronto-nasal and mesial nasal processes later fuse 
completely with each other to form the processus globularis of 
His’s (92 b) descriptions. This globular process then arches 
over the nasal groove and fuses with the maxillary and lateral 
nasal processes, either singly and separately or after those two 
processes have fused with each other, and a nasal bridge is 
formed which is certainly the strict homologue of the bridge in 
the Teleostomi, for the fold of the secondary upper lip can, at 
the most, simply have caused a widening of the bridge. The 
fact that, in the Mammalia, the posterior nasal aperture is 
temporarily closed by the contact of the cut ends of the fold of 
the secondary upper lip, and that a membrana bucco-nasalis is 
formed there and later broken through, is certainly simply a 
modification of the normal process of development, as found in 
the Teleostomi, and is due to the fold of the secondary upper lip 
not passing across the center line of the definitive nasal bridge. 
Keibel also considers this manner of formation of this aperture 
of no morphological significance, for he says (’93, p. 477): 
so erscheint es mir von Wichtigkeit, dass festgestellt wurde, dass der 
laterale Stirnfortsatz an der Bildung des primitiven Gaumens beteiligt 
