472 REIGHARD—PHELPS. [Vou. XIX. 
cent-shaped elevated area seen externally. The dorsal wall of 
this diverticulum is made up of a single layer of columnar 
entoderm cells, which are in contact with the nervous layer 
of the ectoderm. Ventrally the cavity of the diverticulum is 
bounded by the yolk from the surface of which a few cells 
have been segregated. The cavity as seen in longitudinal 
section is crescent shaped. 
b. The Mesoblast—In the trunk region, and as far for- 
ward as the midbrain region, the archenteric cavity is well 
developed and extends across the middle line. In the an- 
terior head region, however, the increased depth of the brain 
causes the dorsal wall of the archenteron to be in contact 
with the yolk in the middle line, and thus obliterates the 
median portion of its lumen. The lateral portions of the 
archenteric cavity in the anterior head region are, on the 
other hand, well developed and connect the archenteric cavity 
of the trunk with the cavity of the crescent which is thus 
shown to be a part of the archenteron. 
In an embryo a few hours younger than the one figured 
the crescent is not readily visible externally,* but may be 
easily found in sections. The columnar entoblast forming its 
dorsal wall is found to be much thickened and many layered 
at its anterior and lateral edges and to become there contin- 
uous with the yolk. The crescent is thus terminated in front 
and laterally by a germinal wall, much better developed later- 
ally than in front. Posteriorly the crescent entoblast is con- 
tinuous with the entoblast forming the dorsal wall of the 
archenteron in the head region. This entoblast of the head 
region is very thin in the middle line beneath the brain where 
it is in contact with the yolk. On either side of the middle 
line, however, it is many layered and very thick and is con- 
tinuous laterally by means of a germinal wall with the yolk. 
Thus in the head region in front of the hindbrain there is as 
yet no mesoblast, but only the much thickened layer of ento- 
blast continuous in front with the columnar entoblast of the 
crescent. About the borders of this entoblast sheet and 
*Such a stage is figured by Beckwith, 1907, Plate I, Fig. 1. 
