200 BULLETIN OF THE 



"cerebral tubes," the deep ends of which are enlarged ('88, Fig. 24). 

 From their inner ends a rapid cell proliferation takes place, the prod- 

 ucts of which join the cerebral cells already in position. The invagi- 

 nated portions later form the " accessory lobes " of the brain. At a 

 late stage only one pair of tubes remains open to the exterior, and the 

 openings to these are closed before the end of embryonic life. The 

 Sarasins ('88, p. 61) do not know the precise time at which they are 

 closed, but are certain that the openings do not persist. They express 

 their belief that the cerebral tubes are homologous with the organs of 

 smell in Annelids, which, according to Kleinenberg's studies on Lopado- 

 rhynchus, also originate as invaginations of the Sinnesplatte, and by 

 cell proliferation furnish a part of the material for the brain. 



Prior to any knowledge of the investigations on Helix by the Sarasins, 

 I found very similar conditions in Limax maximus. In this case, how- 

 ever, there is but one invagination of the ectoderm on each side of the 

 body. It corresponds in position to those described in Helix, being 

 perhaps the equivalent of the upper or larger invagination in that 

 species. 



The invaginations in Limax have the form of shallow pits before any 

 other ganglionic cells are to be seen. The cell proliferation, which re- 

 sults in the production of the main portion of the ganglia, takes place 

 during their ingrowth. Possibly the proliferation from the depression 

 between labial tentacle and upper lip represents what was originally 

 a true invagination, and corresponds to the lower of the two invagina- 

 tions described by the brothers Sarasin. In Limax maximus the ex- 

 ternal openings persist until a late stage, and occasionally even after 

 hatching. Here, also, the invaginations form a lobe of the brain, exactly 

 as in the case of Helix (Sarasin, '87, p. 601). 



Two well developed " Seitenorgane " were found by the Sarasins 

 ('88, p. 54) in Helix Waltoni, situated near each other in the " sense- 

 plate " ; and they think (p. 60) that these may correspond in position 

 to the cerebral tubes of later stages. 



The groups of cells embedded in the ectoderm, from which, in my 

 opinion, the greater part of the nervous system in Limax maximus 

 takes its origin, resemble both in the arrangement of the cells and their 

 histological condition the "Seitenorgane" described by the brothers 

 Sarasin ('88, pp. 53-57). But I have never observed bristles, or other 

 terminal structures, projecting toward the outer world. Moreover, in 

 Limax unmodified ectodermic cells usually lie between these groups of 

 large cells and the outer surface of the bod v. 



