4 Edmund B. Wilson 



preserved in successive grades of alcohol. A weak solution of iodine 

 in alcohol and sea-water also gives beautiful results, but is less certain 

 in its action. For staining I bave used Grenacher's alum-carmine, 

 borax-carmine , picro-carmine and Kleinenberg's haematoxylin. 

 Much the best results are obtained by the use of alum-carmine, but it 

 must be used as quickly as possible , since the gelatinous tissue of the 

 mesoderm is apt to shrink if the object be left too long in aqueous fluids. 

 The tissues were decalcified with very weak nitric or hydrochloric acid 

 in 90 per cent alcohol. For maceration, the Hertvvigs' well-kuown 

 mixture of osmio and acetic acids gives good results. 



I. Structure, developinent and fuiictions of the six short or 

 entodermic fllaiiieuts. 



a) Structure, 



These filaments appear to bave essentially the same structure in 

 ali of the forms which I bave examined , including eleven different ge- 

 nera, and I will select Paralcyonium as type. It is not my purpose to 

 give an exhaustive descriptiou of their structure, but only to describe 

 their general features. 



A transverse section through one of these filaments is shown in 

 fig. 12. The septum, s, consists of a delicate lamella, covered on both 

 sides by a pavement-epithelium. In the base of the latter, upon the 

 ventral side , we see muscle-fibres in cross-section. The lamella may 

 be follo wed out into the filament, where it gradually fades away. The 

 filament is formed by a sudden expansiou of the septum , the cells at 

 the same time undergoing a total change of structure. At the back of 

 the filament the cells assume a columnar form , but stili remain clear 

 and only slightly granular. At the sides and in front they become elon- 

 gated and swollen, and their contents undergoes a great change. The 

 cell-bodies stain deeply, so that in most cases the nuclei become ob- 

 scured. As regards their contents, the cells appear mainly in two forms. 

 In one form the celi is closely packed with deeply stained highly re- 

 fringent spheroidal bodies. In the other form the cell-contents is only 

 slightly granular and is uniformly and intensely stained. Both forms 

 may be easily and completely isolated by maceration in Hertwigs' 

 mixture of osmic and acetic acids, and are then seen to bear a single 

 cilium at the outer end , and to contain in the basai part a nucleus like 

 the ordinary entoderm nuclei. At the base they sometimes appear 



