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 VIII.), 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 

 VIII.) is a nearly sagittal section, slightly oblique, through another egg 

 of such a stage, showing the overhanging antei'ior 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- 

 terou is much more vacuolated than the more recently formed venti'al 

 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 the 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 occu- 

 pies the mid-dorsal region, but is shifted about 45° toward the anterior 



