56 J. B. Johnston. 
plate consists of the thin outer layer alone. As development goes 
on this thin outer layer gradually breaks down and its cells seem to 
scale off, leaving the yolk-laden entoderm cells freely exposed (Figs. 
i 8; 0.10) 12, A@rotdonione 
This disappearance of the mouth plate ectoderm I consider com- 
parable to the rupturing of the pharyngeal membrane so far as its 
ectodermal portion is concerned. The entodermal layer of the mem- 
brane is not ruptured at this time beeause of the mechanical condi- 
mand 
Tic. 14. A. punctatum, stage of tooth formation. Median sagittal section 
of the mouth region. The oral entoderm is drawn out into a slender flattened 
rod and the formation of the mouth cleft will soon take place. A mandibular 
tooth is seen in vertical section and dorsal to the entoderm, the ectodermal 
in-growth in which the maxillary and vomerine teeth will appear is distin- 
guished by the absence of yolk. Between d. max. and d. vom, is the projection 
of entoderm which later meets the nasal sac. Iron hematoxylin, fuchsin. 
tions which lead to the coalescence of the walls of the archenteron. 
The appearance of the oro-pharyngeal cavity as a cleft in the entoderm 
completes the process, which is comparable to the rupturing of the 
pharyngeal membrane in other vertebrate embryos. 
5. Formation of Dental Ridges and Teeth. 
The thickening of the inner layer of ectoderm surrounding the 
mouth-plate mentioned above soon becomes prominent, and in both 
