MUSEUM OF COMPARATIVE ZOOLOGY. 175 



stage of which should be kept warm. I have found that parafine which 

 melts between 50° and 52° C. is better for embedding than that which 

 is harder, for the latter is liable in hardening to cause tlie embryo to 

 crack. 



Sections from 10 to 15 /a thick, and in the oldest stages even thicker, 

 are better than very thin ones. 



The central nervous system of Limax consists of four pairs of gan- 

 glia, — namely, cerebral, pedal, pleural, and visceral, — together with 

 one abdominal ganglion. To these more central ganglia are joined in 

 addition a pair of buccal ganglia, and one mantle or olfactory ganglion. 



To summarize briefly in advance my conclusions : The ganglia arise 

 separately. The components of three of the five pairs are joined to- 

 gether later by commissures. Secondarily-produced connectives^ also 

 serve to join the cerebral ganglia to the pedal, the pleural, and the 

 buccal; the pleural to the pedal and the visceral; and the visceral to 

 the abdominal. The growth of the ganglia is rapid ; they are well 

 formed, and in their ultimate positions by the sixteenth day. The prin- 

 cipal changes from that time until hatching, eight or nine days later, 

 are increase in size, and modifications of the histological conditions. 

 According ^o my observations, all the ganglia, with the possible exception 

 of the pleural, are derived directly from the ectoderm, — the cerebral in 

 part from invaginations, the others exclusively by cell proliferation with- 

 out invagination. The cerebral ganglia are formed by extensive invagi- 

 nations, one on each side of the head region, just below and behind the 

 base of the ocular tentacles. During the invagination a rapid cell 

 proliferation takes place at the deep end of the invaginated portion of 

 the ectoderm, and also at a region of the ectoderm corresponding to 

 the depression between the labial tentacles and the upper lips. The 

 lateral halves of the cerebral mass arise as two separate structures, — 

 each from a double origin, — which are only secondarily joined. This 

 union is the result of outgrowths from each of the ganglia which, 

 uniting, form the cerebral commissure. The invaginations begin a 

 little later than the proliferation of cells which gives rise to the pedal 

 ganglia, and they remain open as narrow tubes until towards the period 

 of hatching, or even later. In one instance they have been found in 

 this condition as late as eight days after hatching. The cerebral com- 



1 In accordance with tlie usage introduced by Lacaze-Duthiers, the term com- 

 missure is employed for the nerve fibres joining the components of a pair of 

 ganglia, and connective for those between ganglia on the same side of the body. 



