48 J. B. Johnston. 
terminal ridge of the neural plate (see Johnston, 1909) the ectoderm 
grows thinner. The wedge-shaped piece of ectoderm immediately 
adjacent to the neural plate will form the hypophysis. Its position 
is accurately indicated in Figs. 1 and 2. When the preoral entoderm 
is more blunt or rounded the hypophysis presents in the earliest 
stages a triangular form in sagittal sections (Fig. 3), one angle 
being directed inward between the terminal ridge of the neural plate 
and the preoral entoderm. When the neural plate rolls up the 
terminal ridge participates in the rising up of the neural folds, and 
en 
Kia. 5. A. punctatum, stage when hypophysis begins active invagination. 
Median sagittal section. Haemalum. 
the distinction between the neural plate and hypophysis at once 
becomes clearly marked (Figs. 2 and 3). From the earliest stage 
the hypophysis rests against the tip of the preoral entoderm. 
While the neural tube is rolling up, the hypophysial ectoderm be- 
comes deeper, so that in sagittal section the depth of the triangle 
becomes greater than the length of its base on the surface (Fig. 3). 
Up to this time the cells within the triangle show no regularity of 
arrangement. Immediately following this it is seen that the deeper 
cells are arranged so as to suggest a short blind sac. From the start 
(Fig. 5) this sac is thicker on the anterior or dorsal side. It has 
