KOFOID: DEVELOPMENT OF LIMAX. 109 
Such evidence as there is seems to lead to the conclusion, that the 
orifice at this later stage, though posterior in position, is derived from 
the mid-ventral opening — the anterior end of the blastopore of an earlier 
stage — by a backward overgrowth of the anterior and lateral margins 
of that opening, the posterior lip of the blastopore being regarded as 
fixed, so that the remnant of the blastopore comes to occupy a position 
corresponding to the posterior lip of the blastopore of the earlier of these 
two epochs. If this is the case, we should expect to find intermediate 
stages showing steps in this overgrowth. The earlier stage, Figure 51 
(Plate VIIL), shows some trace of it, for the pit of invagination has an 
antero-dorsal direction, i. e, in a ventral view the anterior lip of the 
blastopore somewhat overhangs the deeper portion of the invaginated 
layer, whereas the posterior margin rises obliquely to the level of the 
ectoderm. This overgrowth is accompanied by an accumulation of 
mesoderm in the anterior region. Although I have examined hundreds 
of embryos of about this stage, many of them killed especially for the 
determination of this question, very few good illustrations of this over- 
growth have been observed. Figure 53 (Plate VIII.) is a postero- 
ventral view of such a stage, showing the thickened anterior end with 
its velar projections. Occupying the mid-ventral region is an elevation 
which overhangs the site of the mid-ventral blastopore, whose posterior 
region is now marked by a trough-like depression. Figure 54 (Plate 
VIIL) is a nearly sagittal section, slightly oblique, through another egg 
of such a stage, showing the overhanging anterior lip and its contained 
mesoderm cells. Owing to the obliquity of the section the contact of 
the ectoderm and entoderm in the mid-dorsal region is not shown in this 
section. "That such an overgrowth as I have suggested takes place is 
also shown by the conditions found in the later stage itself. 
Figures 56 and 57 (Plate VIII.) show that the dorsal wall of the archen- 
teron is much more vacuolated than the more recently formed ventral 
wall. They also show that there has been an accumulation of the meso- 
derm in this ventral region, and that the cells of the ectoderm are 
smaller in the ventral than in tho dorsal half of the embryo. All of 
these facts seem to point to a more rapid growth in this mid-ventral 
part of the embryo. 
The conditions in the mid-dorsal region are of considerable interest. 
Assuming that the surface of contact between ectoderm and entoderm is 
constant now, as it has been during gastrulation, except when it is inter- 
rupted by the ephemeral cleavage cavity, we find that it no longer oceu- 
pies the mid-dorsal region, but is shifted about 45% toward the anterior 
