178 BULLETIN OF THE 



the ectoderm at the base of the ocular tentacles, at a point immediately 

 below the nephridial organs. At the same time that the infolding takes 

 place, cells, whose nuclei are larger than those of the mesodermic cells, 

 are being proliferated from the deep surface of the invaginating portion 

 of the ectoderm (Plate I. Fig. 6). 



In the region of the ventral wall of the foot referred to in the stage 

 previously described, there are in the ectoderm of each side of the body 

 two groups of ganglionic cells (prf. pd., Plate I. Fig. 7), one behind the 

 other. These cells project into the cavity of the foot, and reach nearly 

 to another small group of cells situated not far from the ventral wall. 

 The cells of the latter group (there is one group on each side of the 

 body) have nuclei similar to those of the cells still connected with the 

 ectoderm. Each group lies in the position subsequently occupied by the 

 pedal ganglion of its side of the body, and is undoubtedly the beginning 

 of that ganglion, for the cells in the ventral wall of the foot continue to 

 be proliferated during several days, and are found in some individuals 

 to be in direct continuity with the ganglia after the latter have at- 

 tained considerable size. In the individuals shown in Figures 7 and 9 

 (Plate I.), the right otocyst (Fig. 7) is seen as a closed vesicle, which is 

 not yet wholly detached from the ectoderm. The otocysts undoubtedly 

 vary in regard to the time of their detachment, as will be seen by a 

 glance at the left otocyst of the same individual, which has entirely 

 lost its connection with the ectoderm (Fig. 9). 



All the other ganglia, with the exception of the one near the olfactory 

 organ and the buccal ganglia, arise by cell proliferation from ectoderm 

 which lies between the foot and the head region, either at or a little 

 above the posterior angle formed by the body wall with the dorsal sur- 

 face of the foot, or along a depression which runs forward from this 

 point. This angle marks the posterior limit of a furrow which passes 

 obliquely forward and downward, partially separating the head and 

 visceral mass from the foot. This depression will be designated as 

 the pleural groove. Of the remaining ganglia, only the visceral have 

 begun to be formed at this time. The cells destined to form these 

 ganglia are situated immediately above the angle produced by the 

 pleural groove (Plate I. Figs. 8 and 9). Some of those of the left 

 ganglion are wholly detached from the ectoderm, but those of the 

 right (Fig. 7) are still continuous with the ectoderm, though pro- 

 jecting into the body cavity. The cells have large, round, faintly 

 stainable nuclei, each containing one large nucleolus, which takes a 

 deep stain. 



