108 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
embryo. The blastoporic trough has now closed gradually from behind 
forward until there is a laterally compressed, cone-shaped pit directed 
antero-dorsally, aud occupying a region at the vegetative pole directly 
opposite that where the polar globules are located, i. e. the blastopore 
at this period occupies a position corresponding to the centre of the 
vegetative half of the egg. 
The “landmarks” which assist in the orientation of this stage are : 
(1) The two lateral massos of mesoderm, now in continuity at the thick- 
ened region anterior to the blastopore, but separated posteriorly in the 
mid-dorsal region, where the ectoderm and entoderm retain their primi- 
tive contact. (2) The posterior region of the mesodermal bands, which 
marks the level of the posterior margin of the blastopore. These lic at 
the smaller end of the embryo. (3) The lateral projections, which may 
well be called “velar” projections, for they occupy a position similar to 
that of the velum in Planorbis. The cells of this region are also somo- 
what larger than those of the surrounding ectoderm (Plate VIII. Fig. 57). 
Owing to their constancy and promineneo they are a valuable aid in the 
orientation of the embryo. 
At a stage succeeding this, represented in sagittal and transverse sec- 
tion respectively in Figures 56, 57 (Plate VIIL), a considerable trans- 
formation has been effected. The whole embryo has increased somewhat 
in size, owing to the growing vacuolation of the entoderm. The velar 
projections still persist and serve to emphasize the difference between 
the anterior and posterior ends of the embryo, as before ; but the rem- 
nant of the blastopore no longer occupies a median ventral position ; it 
is found at the posterior end of the embryo, Figure 55 (Plate VIIL). 
The relation of this to the preceding stage is a matter of considerable 
importance, owing to its bearing on the question of the fate of the 
blastopore. 
It is with considerable hesitancy that the consideration of this per- 
plexing question — the fate of the blastopore — is undertaken, for 1 
annot bring forward to support my conclusions the decisive evidence 
that can be offered in other forms, e. g. Umbrella. There are in Limax 
no definitely fixed and prominent landmarks, such for example as the 
anal cells of Umbrella. The germ layers themselves are not sharply 
differentiated from ono another by the size of the cells, and the whole 
matter is further complicated by tho existence of the recurrent cleavago 
cavity, the beginning of the vacuolation of the entoderm, and the 
development of the head vesicle, all of which contribute to profound 
changes in the external form of the embryo. 
