MUSEUM OF COMPARATIVE ZOOLOGY. 187 



one (Fig. 70) ; the right one is much longer and more attenuated, since 

 the right visceral ganglion is farther from the pleural than the left vis- 

 ceral. The ganglia are most distinctly seen in specimens cut in a 

 sagittal direction. 



The visceral ganglia (Plate V. Figs. 67-69, Plate VI. Fig. 70) are 

 much larger and more elongated in the direction of the pleural ganglia — 

 i. e. downward, forward, and outward — than they were during the pre- 

 vious stage. They are still connected with the ectoderm at their pos- 

 terior dorsal ends, while the opposite ends are much drawn out toward 

 the pleural ganglia (Figs. 69, 70). The right visceral ganglion (Figs. 

 67-69) is larger than the left, and its longest axis has a dorso-ventral 

 direction (Fig. 68). The fibrous prolongations continue into the pleuro- 

 visceral connectives (Fig. 71). 



The abdominal ganglion (Plate III. Figs. 43, 44, 46, 47, Plate VI. 

 Figs. 75, 76), although still connected with the ectoderm, is also larger, 

 and projects more into the body cavity than on the tenth day. A large 

 portion of it still lies to the left of the median plane of the body (Plate 

 VI. Figs. 75, 76), and the connective to the left visceral is well devel- 

 oped (Plate III. Figs. 41, 42, Plate V. Fig. 68); that to the right is 

 less complete (Plate III. Figs. 45, 51). 



The buccal ganglia (Plate V. Fig. 62, Plate VI. Fig. 77) are now 

 very distinct ; the dorsal wall of the radula sac still contributes to their 

 increase in size. 



Cell proliferation takes place from the ectoderm bordering the en 

 trance to the respiratory cavity. A few cells, which probably form the 

 olfactory ganglion, are seen at this stage to be separating from the ecto- 

 derm in this region. 



For the next twenty-four to thirty-six hours (twelfth and thirteenth 

 days) the external appearance of the embryo remains nearly the same 

 as on the eleventh day. In the living embryo the larval heart may be 

 seen pulsating, and the foot gland extends somewhat farther towards the 

 posterior extremity of the foot. 



The cerebral invaginations appear simply as long narrow sacs filled 

 with a coagulated substance ; the inner ends of these sacs have grown 

 upward as well as backward (Plate VII. Fig. 94). The proliferated 

 portions of the cerebral ganglia (Fig. 94) are much larger, and have 

 now assumed more nearly their ultimate positions (Plate III. Figs. 48, 

 49; Plate VII. Figs. 81, 82, 94). The central portion of each has 

 become more fibrous (Fig. 81). 



