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, and 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 masses 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 lie 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 some- 

 what larger than those of the surrounding ectoderm (Plate VIII. Fig. 57). 

 Owing to their constancy and prominence 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 VIII.), 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 VIII.). 

 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 I 

 cannot 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 one another by the size of the cells, and the whole 

 matter is further complicated by the existence of the recurrent cleavage 

 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. 



