470 MEAD. [Vor 1X 
Soon, however, the mesoderm leaves the surface and sinks 
into the segmentation cavity as described above. As it does so 
the somatic plate extends backward and meets the entoderm 
plate. This slight movement carries the morphological pos- 
Figiil.(A. Or 
bep. 
terior end of the animal to a position 180° from the apical pole 
(Fig. III). At this point it remains, so that now and henceforth 
the antero-posterior axis coincides with the vertical axis. 
As soon as the mesoderm has sunk into the segmentation 
cavity the entoderm plate also begins to invaginate. The cells 
gradually elongate, so that the entoderm plate becomes thick- 
ened, while its surface area diminishes. This process continues 
until at length the entoderm cells withdraw entirely from the 
surface and lie within the segmentation cavity, and the blasto- 
pore has closed. 
At the same time with the thickening of the entoderm plate 
and the diminution of its surface area, there goes the thinning-out 
of the somatic plate by successive cell 
divisions and a lateral extension of its 
surface area. In fact, the lateral edges 
of the somatic plate follow the retreat- 
ing boundary line of the blastopore, 
and, when the blastopore has closed, 
meet in the mid-ventral line, uniting 
with each other by a process of typical 
concrescence, and separate the blasto- 
pore from the terminal cells (Fig. IV). 
Long before the closure of the blastopore the paratrochal 
cells are fully differentiated, and enclose the terminal cells, 
