No. 2.] THE ADHESIVE-ORGAN OF AMIA. 477 
The pericardial cavity has developed in this mesoblast 
sheet and may be seen in Plate, Fig. 2, extending backward 
between the outermost white line on the yolk and the branch- 
ial area to become continuous with the body cavity further 
back. Mesoblast also occupies the space between the ad- 
hesive organ and button on the one hand and the optic ves- 
icles and brain on the other (Fig. B, mes.). Examination of 
intervening stages shows that the mesoblast of the region an- 
terior and lateral to the adhesive organ has been produced, 
largely if not wholly, by progressive forward differentiation 
from the germinal wall. This germinal wall extends along 
the borders of the entoblast of the head and trunk region and 
forward along the lateral and anterior borders of the colum- 
nar entoblast of the crescent. The germinal wall of the 
lateral borders of the crescent is especially well developed 
and from it the mesoblast extends outward and also inward 
and backward toward the head, until it becomes finally contin- 
uous with the mesoblast previously formed in the head region. 
The mesoblast at the anterior end of the brain and optic 
vesicles and between them and the adhesive organ and button 
owes its crigin in part to proliferation from the button. The 
middle portion of the posterior surface of the button is in con- 
tact with the hypophysis (Fig. C) and no mesoblast «is pro- 
liferated from this portion of its surface at this stage. At 
the sides of the hypophysis on the other hand the entoblast 
of the button may be seen to be in process of transformation 
into mesoblast. Cells here separate themselves from the but- 
ton and at the same time assume the form of stellate mesen- 
chyme. The whole button is thus finally affected so that by 
the next stage it is no longer recognizable. The crescent 
has given rise not only to the adhesive organ, but to a 
considerable amount of mesoblast which is proliferated from 
that portion of the germinal wall forming its borders as well 
as from the button. The mesoblast of the head is then seen 
to be formed principally by delamination from the primi- 
tive entoblast, but partly also by proliferation from the cres- 
cent in the manner just indicated. 
