180 BULLETIN OF THE 



before. The cells of the mesoderm form a continuous layer along tho 

 inner surface of the ectoderm, except where cell proliferation is taking 

 place (Figure A). 



As yet nothing is to be seen of the pleural ganglia. 



The visceral ganglia have increased in size (Plate I. Figs. 10, 11, 12); 

 they are still connected with the ectoderm (Figs. 10, 12), although a few 

 cells with large nuclei have become detached from it (Fig. 11). The 

 ganglion and the otocyst of the same side of the body lie in nearly the 

 same sagittal plane. Each ganglion is situated just above the angle 

 caused by the pleural groove. The right visceral ganglion (Fig. 10) is 

 somewhat farther forward and more dorsal than the left (Figs. 11, 12). 



About in the median plane of the body, and above the angle made by 

 the pleural groove, are the cells which form the abdominal ganglion 

 (Plate I. Fig. 13). The greater part of them are still embedded in the 

 ectoderm. Although in some regions they project into the body cavity, 

 they are nowhere wholly separated from the ectoderm. The abdominal 

 ganglion seems to be at first more intimately connected with the left 

 visceral ganglion than with the right, but a connective is formed with 

 both of them a little later, and the abdominal ganglion thus appears to 

 occupy the place of a direct commissure between the two visceral ganglia. 

 As development proceeds, the abdominal ganglion becomes closely fused 

 with both the visceral ganglia. 



Quite an advance in external conditions is made by the ninth day. 

 But individuals of the same age vary so much in the degree of develop- 

 ment attained by both their external and internal organs, that the age 

 assigned can be taken only as an approximation to the average condition 

 at the time indicated. 



The tentacles appear as protuberances, the labial tentacles being much 

 smaller than the ocular ; the shell gland contains more concretions, the 

 mantle is larger and bends backward over the dorsal surface of the foot. 

 The radula sac makes its appearance and extends backward into the foot, 

 where it ends blindly immediately back of the pedal ganglia. In trans- 

 verse sections it appears flattened dorso-ventrally ; its lumen is oval, and 

 the ectoderm lining it is more than one cell deep. 



The cerebral invaginations (Plate II. Figs. 15, 19, Plate III. Figs. 25, 

 26) are much deeper, the infolding ectoderm is greatly thickened, and 

 the incipient ganglia receive accessions from ectodermic depressions be- 

 tween the rudiments of the upper lips and the labial tentacles (Plate II. 

 Fig. 21). The cerebral commissure (Fig. 21) is also being formed, the 



