182 BULLETIN OF THE 



form the respiratory chamber (Plate II. Figs. 1G, 24), the region from 

 which the ganglionic cells arise is now located on the ventral and median 

 walls of the infolding. The ganglia have also grown forward, and lie 

 between the nephridial organs and the nutritive sac (Plate II. Fig. 23). 

 In an individual cut crosswise, the posterior portion of the ganglia is 

 found to be two or three sections back of the otocysts. Both the 

 ganglia may be traced through five or six sections. 



A little behind the visceral ganglia, and to the left of the median 

 plane of the body, are the prominent cells of the abdominal ganglion 

 (Plate II. Figs. 24, ab.). 



All of these ganglia still consist of groups of loosely associated cells. 

 Later they become more compact, and are surrounded by connective- 

 tissue cells. 



The buccal ganglia (Plate II. Fig. 22), first seen with certainty at 

 this stage, arise, one on each side of the radula sac, at the angle between 

 it and the oesophagus. It is to be seen from cross sections that the cell 

 proliferations from which they spring take place from the dorsal wall 

 of the neck of the sac, where its lumen begins to be separated from that 

 of the oesophagus. This is also their permanent position ; they are later 

 joined together by a commissure, which results from outgrowths of the 

 cells composing the two ganglia. 



On the tenth day the external appearance of the embryo remains 

 nearly the same as before, with the exception that there is an increase 

 in the size of the embryo, and especially of its pulsating sacs. The sac 

 of the radula has become more elongated, and the anal opening (Plate 

 III. Fig. 31, an.) is formed. 



The cerebral invaginations still appear, in sections parallel to the 

 sagittal plane (Plate III. Figs. 28 and 29), as shallow depressions. The 

 number of cells in each ganglionic group (Plate IV. Fig. 58, Plate Y. 

 Fig. 63) has increased perceptibly. At the same time the groups have 

 extended backward, and show indications of the cerebro-pleural connect- 

 ives. In specimens cut in the sagittal plane, the cerebral commissure 

 cut crosswise may be seen above the oral opening (Plate III. Fig. 30). 



The pedal ganglia (Plate IV. Figs. 54, 58, Plate V. Fig. 63) have in- 

 creased in size. Their anterior borders now reach as far forward as the 

 plane of the pleural groove, and they extend backward into the foot 

 much farther than before. In cross sections (Plate IV. Fig. 54) they 

 appear as rounded groups of cells, which are far apart and not yet very 

 compact ; they still continue to receive accessions by the proliferation of 



