186 BULLETIN OF THE 



are lodged the larger scattered cells with their very large nuclei, and a 

 peripheral part, where the cells are crowded together and the nuclei 

 are smaller (Fig. 70). They are loosely enveloped by spindle-shaped, 

 very much elongated, connective-tissue cells (Fig. 71). Immediately 

 above the oral cavity is the cerebral commissure (Plate VI. Fig. 80 a ). 

 It can be traced from one side of the brain to the other, and its cross 

 section appears as a very small round patch of fibrous substance, sur- 

 rounded on the dorsal side by a layer of flat cells. 



The cerebro-pedal connectives are indicated (Fig. 71) by a few cells 

 extending from the ventral-posterior ends of the cerebral ganglia to the 

 anterior ends of the pedal, a little in front of the cerebro-pleural con- 

 nectives (Fig. 70). The latter extend from the posterior ends of the 

 cerebral to the anterior ends of the pleural ganglia, thus diverging 

 somewhat from the cerebro-pedal connectives. There are found in the 

 ganglia many cells which are in different stages of division. It is owing 

 to this cell division that the ganglia increase rapidly in size, especially 

 after they are wholly cut off from the ectoderm ; cells in the commis- 

 sures and connectives are also found in process of dividing in planes 

 perpendicular to the direction of their fibres. 



The principal change in the pedal ganglia (Plate VI. Fig. 71) is due 

 to an increase in size, particularly in the antero-posterior direction. The 

 central portion of these ganglia has the same fibrous appearance as that 

 described for the cerebral ganglia, and the pedal nerves can be traced for 

 a considerable distance toward the tip of the foot (compare Figure E, 

 page 191). The anterior commissure (Plate III. Fig. 44, Plate VI. 

 Fig. 74) is now somewhat shorter than in the previous stage, and con- 

 sists of a greater number of cells. Cell proliferation is still taking 

 place from the ectoderm of the ventral wall of the foot (Plate VI. Fig. 

 71), and the ganglia continue to receive accessions from these sources. 

 More highly magnified views of the regions of proliferation are given in 

 Plate VI. Figs. 72 and 73. 



The pleural ganglia (Plate VI. Fig. 70) are now easily recognized. 

 Each ganglion is formed of a triangular group of cells, occupying a posi- 

 tion immediately above and anterior to that part of the pleural groove 

 which is nearest to the otocyst. The cells composing the ganglion are 

 fewer than those of any of the other pairs of ganglia, but resemble them 

 in their histological conditions; they are only loosely connected, and 

 their fibres are elongated in the directions of the three connectives. At 

 this stage the ganglia are not closely enveloped in connective tissue. 



The pleuro-visceral connectives are well developed, especially the left 



