MUSEUM OF COMPARATIVE ZOOLOGY, 117 
Larvenleibes ändert sich jedoch allmählig das frühere Verhältniss zu 
Gunsten der Entodermfüllung, die noch immer keine wahre Höhle, sondern 
eine schmale lineare, mit der Hauptachse des Leibes zusammenfallende Spalte 
besitzt.” |} With the growth of the entodermal layer, the coelenteron 
enlarges, and the cleavage cavity is diminished, until finally it is 
entirely obliterated and the entoderm everywhere comes into contact 
with the ectoderm (Plate I. Figs. 4-6, Plate II. Fig. 11). 
During the process of gastrulation, and also for a short time after its 
completion, the thickness of the entoderm, which is much less than that 
of the ectoderm, does not increase. Figures 5 and 6 (Plate I.) are 
from ‘sections of two embryos at different stages of development. 
Figure 5 is from an embryo soon after the completion of gastrulation ; 
Figure 6 is from an older stage. Since in each case the section is from 
the middle of its series, it follows that a decided thickening of the ento- 
derm takes place between the stages represented by these Figures. This 
thickening is apparently due to an increase in the number of the cells, 
which are soon unable to find room for themselves except by elongation. 
The entodermal cells are quite different in appearance from those of 
the ectoderm; they are,approximately spherical, and do not have as 
numerous spheroidal yolk bodies as the latter. Their nuclei, however, 
closely resemble those of the ectoderm, and usually lie in the portion of 
the cell nearest the coelenteron. 
As is to be seen from Plate II. Fig. 7,— a section nearly perpendicular 
to the blastoporic canal, — the blastopore in A. flavidula is very small. 
A similar condition has been shown by Claus to exist in A. aurita and 
by Metschnikoff (86, Taf. X. Fig. 14) in Nausithoe marginata. 
The nuclei of the cells composing the wall of the blastosphere are sit- 
uated, as has been stated, near the surface of the sphere. But at about 
the time of the beginning of the invagination, sometimes a little earlier, 
a few of the nuclei are found in the deeper portion of the wall. At first 
there are only one or two such displaced nuclei to be observed in the 
whole embryo, but as development progresses they increase in number. 
A careful examination of sections shows that the cells to which they 
belong do not extend, like the remaining cells of the wall, through its 
whole thickness, but that they are wedged in as it were between the 
bases of the ordinary cells. The latter are much elongated, and from 
mutual pressure are prismatic, whereas the deep cells are spheroidal and 
project in some cases into the segmentation cavity. Since these cells 
are found at various intermediate positions between the outer and inner 
1 The original is rot Italicized. 
