176 BULLETIN OF THE 



missure is formed a littte earlier than the commissural fibres joining 

 the pedal ganglia. The latter are connected by two distinct commis- 

 sures, the anterior of which is formed earlier than the posterior. The 

 visceral ganglia precede a little in their development the pleural, 

 abdominal, buccal, and mantle ganglia. The buccal ganglia make their 

 appearance at about the same time as the pleural, and undergo almost 

 no change in position. 



The nervous system in Limax maximus makes its appearance on the 

 sixth or seventh day after the egg is laid. At this time the foot is a 

 conical projection, less than half as long as the diameter of the more 

 or less spherical remaining portion of the embryo, and its pulsating sac 

 is very small. It is a stage which is only slightly older than that rep- 

 resented by Fol in his Plauche 17-18, Fig. 7. The ocular tentacles 

 are now distinguishable as small elevations of the head region, near 

 the beginning of the primitive nephridial organs, but the labial tenta- 

 cles are barely to be made out. The radula sac is a nearly spherical 

 outfolding of the floor of the oral sinus ; its fundus is composed of only 

 a single layer of cells, but the part of the sac which is continuous with 

 the wall of the CEsophagus is more than a single cell deep ; the lumen 

 of the oesophagus is traceable close up to the yolk, where it ends 

 blindly. Both the oesophagus and the radula sac are covered with a 

 continuous layer of somewhat flattened mesodermic cells. The shell 

 gland has the form of a large thin-walled sac containing concretions. 



When this condition has been reached, the head region (Plate I. 

 Fig. 2) exhibits no sign of cerebral invaginations, nor have I been able 

 to find regions of cell proliferation or thickenings in the ectoderm which 

 were referable with certainty to the cerebral ganglia. 



So far as I have been able to make out, the first contribution to the 

 formation of the pedal ganglia occurs in the form of small clusters of 

 cells, which are still imbedded in the ectoderm of the ventral wall of the 

 foot (Plate I. Fig. 5), from which they are subsequently detached. 

 Each of these clusters has a spheroidal or more ridge-like form, and con- 

 tains from four to eight cells. The boundaries of the cells are not 

 sharply marked, but the whole cluster is limited by a definite outline 

 separating it from the rest of the ectoderm. Each cell contains a nu- 

 cleus, which is large, but less deeply stained than those of tne ectoderm, 

 and each nucleus has a large nucleolus, which is very deeply stained 

 (Plate T. Fig. 1). 



The region in which this proliferation takes place is definitely located, 



