The Limit Between Ectoderm and Entoderm. AT 
longest in connection with the entoderm is marked by a sharp notch 
(see Johnston, 1906, Fig. 36). In front of this the entoderm is not 
thick as in selachians so that the term “median mass” is not so appli- 
cable. However, from this part of the entoderm mesenchyme splits 
off and leaves the entoderm very thin, often scarcely a continuous 
membrane. This thin area is soon filled up by the shortening and 
p cm. 
IG. 45 A Jane, 2E 18 
Fic. 4. A. punctatum, two parasagittal sections to show the relations of the 
head mesoderm. Hzemalum. 
shifting back of the preoral entoderm and by the general compression 
which soon obliterates the anterior end of the archenteric cavity. 
2. Development of Hypophysis. 
The ectoderm which will form the hypophysis can be recognized 
as soon as the neural plate is formed (Figs. 1, 2, 3). Below the 
