474 E. A. MINCHIN. 



characteristics of the cells composing the wall of the sponge. 

 I propose, therefore, to describe the histology in sufficient 

 detail for the aim in view, reserving fuller treatment of the 

 various issues for another paper. 



(a) Methods of Investigation. — For studying the de- 

 velopment of the spicules, and the general arrangement and 

 characters of the cell elements composing the body-wall of 

 an Ascou, no very elaborate histological technique is required ; 

 in fact, almost everything can be made out from surface views 

 of pieces of the body-wall fixed with osmic acid, stained with 

 picrocarmine, and laid out flat in glycerine. There is, how- 

 ever, one precaution, the importance of which cannot be too 

 strongly emphasised : the sponges should not be brought back 

 to the laboratory and there preserved, but should be fixed 

 quite fresh from their habitat immediately they are found, 

 care being taken to select fully expanded specimens. 



My method of operating is to carry with me, when seeking for 

 the specimens, different tubes containing osmic acid solution, 

 distilled water, and picrocarmine, or other reagents as may 

 be required. I take 1 per cent, osmic solution, and dilute 

 it with an equal volume of sea water. The picrocarmine 

 employed is either Ranvier's or Weigert's, both obtained from 

 the well-known firm of Griibler at Leipzig. Ascons are very 

 easy to preserve, even in liquids of very feeble penetrating 

 powers, on account of the numerous cavities and channels, 

 resulting from their structure as systems of thin-walled tubes. 

 Hence quite large specimens can be fixed whole in osmic 

 without fear. After being five to ten minutes in osmic the 

 specimens are rinsed with water and placed in the picro- 

 carmine. In this stain they can be brought back to the 

 laboratory, but should not stay in it more than two hours ; in 

 general one hour is sufficient. The specimens are then washed 

 with distilled water and transferred to glycerine or alcohol, 

 according as surface preparations or sections are required. 

 As a rule I put one half of each specimen so preserved into 

 glycerine, the other half into alcohol. 



This method preserves the cells, so far as shape, appearance, 



