MUSEUM OF COMPARATIVE ZOOLOGY. 181 



cells of the median portion of each ganglion growing out to meet the 

 corresponding cells from the opposite ganglion. The commissure at this 

 stage is composed of a small number of cells, which are very much elon- 

 gated. The fibres resulting from their elongation already make a con- 

 tinuous bridge from one ganglion to the other. 



The pedal ganglia (Plate II. Fig. 20, Plate III. Fig. 27, Plate V. Fig. 

 60) consist of two small groups of cells, situated about midway be- 

 tween the sole of the foot and the posterior end of the radula sac. They 

 are a little below and behind the pleural groove and the otocysts, and 

 they are farther from each other than from the lateral wall of the foot. 

 There is a slight indication of a commissure (Plate III. Fig. 27) joining 

 their anterior portions to each other. The commissure is formed in the 

 same manner as the cerebral commissure, the individual cells composing 

 it being spindle-shaped, with their nuclei somewhat elongated in the 

 direction of the fibres. 



The otocysts (Plate II. Fig. 20, Plate III. Fig. 27, Plate V. Fig. 60) 

 are on a level with the lower margin of the radula sac, and are nearer the 

 pedal ganglia than in the preceding stage. 



On each side of the body above the pleural groove is a group of a 

 few cells, which are in all probability the first indications of the pleural 

 ganglia (Plate II. Figs. 14 and 20). The centre of each cluster is seen 

 on cross sections (Fig. 20) to be nearly on a level with the lumen of the 

 radula sac. The cells at this stage are very small, and so loosely associ- 

 ated that it is difficult to distinguish them from mesodermic cells. I 

 have not satisfactory evidence of their origin directly from the ectoderm, 

 for, although I have found them at times very near to the ectoderm 

 (Fig. 20), I have never found them at any stage continuous with it. On 

 the other hand, I have not seen conditions which would warrant the 

 conclusion that the ganglia were the result of outgrowths from either of 

 the pre-existing ganglia. 



A little before the ninth day the cells detached from the ectoderm 

 to form the visceral ganglia (Plate II. Figs. 17, 18) increase rapidly in 

 size, and the diameter of their nuclei often becomes four or five times as 

 great as that of the ectodermic nuclei. The ganglia consist of elongated 

 groups of such cells, still attached to the ectoderm above the pleural 

 groove (Figs. 16, 18). The want of symmetry in the positions of the 

 right and left ganglia is more conspicuous than in the preceding stage, 

 the ganglion of the right side being considerably more dorsal and far- 

 ther back than that of the left side (Plate II. Fig. 23, Plate V. Fig. 60). 

 Owing to the infolding of the ectoderm on the right side of the body to 



