The Limit Between Ectoderm and Entoderm. 59 
nasal sacs have been formed and the wall of the nasal sac comes into 
contact with the elbow of entoderm lateral to the maxillary and 
vomerine teeth. The pharyngeal cleft invades this part of the 
entoderm first, and a connection is established between the cavity of 
the nasal sac and the oro-pharyngeal cavity (Fig. 17). In this 
way the alimentary canal has two openings to the exterior through 
the nasal saes for one or two days before the mouth is open. When 
the mouth cleft is finally completed the entoderm lines the oral 
—_ 
~ nasal sae 
Fic. 17. Parasagittal section from the same series as Fig. 16. The section 
passes through the opening of the nasal sae into the oral cavity. 
cavity to the extreme borders, and even extends out slightly on the 
surface (Figs. 16, 17). Now the maxillary, mandibular and vome- 
rine teeth pierce the entoderm and enter the mouth cavity. 
In the series of embryos used for this study the limits of the 
entoderm are clearly visible for a period of about ten days after the 
mouth cleft was formed. (The rate of growth was slow as the 
running water in the laboratory was colder than that in the ponds.) 
The entoderm during this period is thicker than the ectoderm, con- 
tains numerous yolk granules which are lacking in ectoderm and is 
more opaque in appearance in sections. The entoderm lines the 
