The Limit Between Ectoderm and Entoderm. 45 
are taken from sections which show the slightest tearing or breaking 
in the parts concerned. 
1. Early Form and Relations of Entoderm. 
After completion of gastrulation the archenteron has the well 
known form characteristic of amphibian embryos: The enlarged 
anterior part of the archenteron is bounded in the region where the 
mouth will form by a relatively thin layer of entoderm and a thinner 
layer of ectoderm (Fig. 1). In this region are two broad, very 
shallow depressions in the ectoderm. The more anterior one repre- 
sents the hypophysis, the posterior one the future mouth. Opposite 
these the archenteron itself presents two prominent pits (or angles) 
as seen in sagittal sections. The anterior one is a slender pointed 
S|. 
n eb 
Fic. 1. Amblystoma punctatum, neural plate stage, sagittal section of mouth 
region. Borax carmine stain. 
cavity (Fig. 2), and the entoderm surrounding it forms a blunt 
wedge projecting between the neural plate and the ectoderm. ‘The 
posterior one is a broad depression which corresponds to the future 
mouth opening. The entoderm surrounding the anterior pit is 
the preoral entoderm and corresponds in every way to the preoral 
entoderm of most selachians (cf. Johnston, 1909). 
