The American journal of anatomy . the pharyngealcavity progresses toward the mouth it advances most rapidly towardilic lateral angles of the mouth. When the tooth germs begin toform there is a thin place left in the ectoderm between the maxillaryand vomerine teeth. At this place the entoderm forms an elbowdorso-cephalad which is quite pronounced at the lateral border of(lie tooth-forming areas (Figs. 14, 17, 18). In the meantime the The Limit Between Ectoderm and Entoderm. 59 nasal sacs have been formed and the wall of the nasal sac comes intocontact with the elbow of entoderm lateral to the m
The American journal of anatomy . the pharyngealcavity progresses toward the mouth it advances most rapidly towardilic lateral angles of the mouth. When the tooth germs begin toform there is a thin place left in the ectoderm between the maxillaryand vomerine teeth. At this place the entoderm forms an elbowdorso-cephalad which is quite pronounced at the lateral border of(lie tooth-forming areas (Figs. 14, 17, 18). In the meantime the The Limit Between Ectoderm and Entoderm. 59 nasal sacs have been formed and the wall of the nasal sac comes intocontact with the elbow of entoderm lateral to the maxillary andvomerine teeth. The pharyngeal cleft invades this part of theentoderm first, and a connection is established between the cavity ofthe nasal sac and the oro-pharyngeal cavity (Fig. 17). In thisway the alimentary canal has two openings to the exterior throughthe nasal sacs for one or two days before the mouth is open. Whenthe mouth cleft is finally completed the entoderm lines the oral o ph £5 • • .. *- d. rria nd. Fig. 17. Parasagittal section from the same series as Fig. 16. The sectionpasses through the opening of the nasal sac into the oral cavity. cavity to the extreme borders, and even extends out slightly on thesurface (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 theentoderm are clearly visible for a period of about ten days after themouth cleft was formed. (The rate of growth was slow as therunning 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 ismore opaque in appearance in sections. The entoderm lines the GO J. I!. Johnston. whole on--pharyngeal cavity, meets the ectoderm about midway onthe anterior and posterior surfaces of the branchial arches in thegill slits and meets the ectoderm in fr
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1910
