58 J. B. Johnston. 
the mouth eavity by a layer of entoderm. The eruption of the teeth 
involves the piereing of this entoderm (Figs. 14, 16, 18, 20), 
6. Appearance of Oro-Pharyngeal Cavity. 
The splitting of the solid entoderm takes place progressively from 
behind forward at a stage after the teeth are well formed in the 
dental ridges (Figs. 14, 16, 17). By the time the splitting of the 
oral entoderm takes place the mouth region has grown wider from side 
Tia. 16. A. punctatum, appearance of mouth cleft. Median sagittal section 
of the mouth. The ectoderm and dental ridges are more deeply shaded. The 
yolk granules are all drawn and the outline between ectoderm and entoderm 
is taken from the cell boundaries. Iron hematoxylin, fuchsin. 
to side in anticipation of the future wide mouth. The whole anterior 
part of the entoderm has taken part in this widening, so that the 
oral entoderm is now a broad flat solid plate. As the pharyngeal 
cavity progresses toward the mouth it advances most rapidly toward 
the lateral angles of the mouth. When the tooth germs begin to 
form there is a thin place left in the ectoderm between the maxillary 
and vomerine teeth. At this place the entoderm forms an elbow 
dorso-cephalad which is quite pronounced at the lateral border of 
the tooth-forming areas (Figs. 14, 17, 18). In the meantime the 
