52 J. B. Johnston. 
and preoral entoderm farther and farther back, until they “come 
to lie close to the end of the notochord. Compare Figs. 3 and 10. 
It is this movement of the preoral entoderm dorso-caudally that was 
referred to above as the means of thickening up the wall of the archen- 
teron which is left so thin by the splitting off of the head mesen- 
chyme. The hypophysis finally comes to lie on the dorsal surface 
of the preoral entoderm, and the notch which represents the original 
preoral cavity is visible up to the stage when the velum transversum, 
epiphysis, paraphysis and the inferior lobes of the brain are all 
well formed (Fig. 10). The formation of the saccus vasculosus 
Fic. 10. A. punctatum, stage after formation of epiphysis. Median sagittal 
section. The palzostomal recess is a deep angular pit. The dental ridges: 
are growing in. Heemalum. 
and the differentiation of other structures in the region of the inferior 
lobes takes place considerably later. 
The shifting caudad of the preoral entoderm and hypophysis which 
I have attributed to the brain flexure is accompanied by a very con- 
siderable elongation of the mouth entoderm. While the shifting is 
taking place the mouth entoderm is compressed into a solid mass, 
and this becomes elongated into a comparatively slender cord, flat- 
tened dorso-ventrally (Figs. 10, 12, 14, 16). 
For some time after its separation from the ectoderm the hypophy- 
sis continues to have the form of a shallow cup with the convexity 
