MUSEUM OF COMPAKATIVE ZOOLOGY. 



185 



their appearance during this stage as a pair of evaginations of the 

 lateral walls of the CESophagus, immediately above its communication 

 with the radula sac, and a little in front of the buccal ganglia (Plate 

 VI. Figs. 77-80). 



The cerebral invaginations still open broadly at the sides of the head 

 (Plate III. Figs. 32-34, and Figure C). They are, however, quite deep, 

 and in a series of sagittal sections the depression becomes deeper and 

 deeper as one approaches the median plane, and at the same time the 

 orifice which leads to the depression becomes narrower and narrower, 



PiouRE C. — The posterior face of a transverse section from an embryo of the eleventh day. x 73. 



ab. Abdominal ganglion. 



cav. mt. Mantle cavity 



cnck. Shell gland. 



iv. ceb dx. Right cerebral invagination. 



mt. Mantle. 



rod. Radula sac. 



sul. plu. Pleural groove. 



until it is almost slit-like (Figs. 32-40). The deep ends of the invagi- 

 nation are turned a little towards the median plane. These invagi- 

 nated portions of the brain are composed of small, closely packed ceJls, 

 whose nuclei stain deeply. The proliferated portions of the cerebral 

 ganglia, which are deeper than the sacs (Plate V. Fig. 64, Plate VI. 

 Figs. 70, 71), extend toward ^ach other in the median plane, and back- 

 ward and downward toward the pedal ganglia (Fig. 71). They have 

 now become differentiated into a fibrous central part (Fig. 71), in which 



