MisceUaneous. 187 



investigation, the knowledge which wc possess ahout it must still 

 be l(>nne(l incomplete. This is to a certain extent due to the fact 

 that the majority of the treatises dealing with the (|iiestion belong 

 to a period at which tlic methods of investigation were not suffi- 

 ciently developed, and when, moreover, many questions, the solution 

 of wliich is to-day a matter of the first importance, were as 5-et 

 entirely untouched, llecent writers have satisfactorily filled up a 

 portion of these gaps in the development of the Gastropods, yet 

 many a question — and this apjjlies especially to the Pulmonata — 

 still awaits its solution. I had the opportunity of collecting and 

 examining a rich material of embryos of different terrestrial Pul- 

 monates, and I purpose to give in the following pages a brief account 

 of certain results of my investigations, which are not yet completely 

 concluded, concerning the development of the central nervous sijsttm. 

 An exhaiistive account of the development of this, as well as of the 

 remaining systems of organs, will, however, be reserved for a subse- 

 quent publication, in which my statements, supported by figures, 

 shall be compared with those already to be found in the literature 

 of the subject. 



I investigated the development of the following forms : — Succ'.nM 

 piitris, L., Clausilin laniinata, Mont., and a few other S])ceies of the 

 same genus; Limax cinereo-nir/er a^)d L. (x/restis, Jj. Excluding a 

 few deviations, the forms mentioned agree well together in their 

 development. The statements in the present paper refer to Lima,v 

 agrestis and Clausilia laminafa. 



The entire central nervous system arises by 2:)7-oHferatio7i of the 

 external epithelium of the bodij, and is therefore exclusively ectodermal 

 in origin, a fact which agrees with all reliable statements of recent 

 investigators of Gastropod embryology. I preface my account of 

 the origin of the several pairs of ganglia with a short description 

 of the epithelium of an embryo at the corresponding stage of deve- 

 lopment. 



The epithelium of the young and still spherical embryo cojisists, 

 with the exception of four regions of the body which will be men- 

 tioned forthwith, of large cubical cells, the protoplasm of which is 

 only very slightlj- stained by the reagents employed by me (alum- 

 carmine and ha'matoxylin). On both sides of the wide oral 

 opening, however, the epithelium is composed of close-packed 

 cylindrical cells, which are considerably smaller and at the same 

 time relatively elongate, and take a deep stain ; these regions of the 

 body therefore appear by condensed light as two oval, subsequentlj' 

 reniform, sharply circumscribed disks, the ^'' sensory plates." Be- 

 hind the oral region, corresponding to the subsequent ventral sur- 

 face, there extends a roundish area, the cells of which are entirely 

 similar to those of the sensory plates : this disk of cells soon pro- 

 jects as a blunt cone : it is the earliest rudiment of the foot. Border- 

 ing upon this, and extending on to the dorsal surface, we find a 

 similar circular disk of cells — the rudiment of the mantle, ivith the 

 shell-gland. 



In the course of the further development the whole of the super- 



