289 



for 'step clianged into the cubic or flat one. There is thus no 

 question that the larval epithelium disappears ; it is simply rno- 

 dified cell for cell. And the gland-cells , who in the larva helped 

 to fix the aniraal , secrete in the adult sponge the slimy substance , 

 which covers the whole surface and which is characteristie for 

 Myxilla and other spon ges. 



By-and-by there appear fissures in the inner mass, close un- 

 der the free surface ; these fissures become wider , are lined by 

 endothelium and give rise to the well-known subdermal cavities. 

 A little later other canals and the flagellated chambers appear 

 in the satne way. I never could see anything of their origina- 

 ting from one central cavity , the epithelium of which is repea- 

 tedly described as entoderm. After the appearance of the flagel- 

 lated chambers, the spicules arrange themselves into bundies and 

 the first traces of spongine become visible. 



It might be useful to give a short account of the methods 

 foliowed. 



The freely swimming larvae were put into a glass , the bottom 

 of which was covered by a loose, thin sheet of collodium , on which 

 they attach themselves readilly. The place where a larva was 

 fixed was then cut out under water at various tiraes after the 

 attachment , and the larvae thus together with the collodium , pre- 

 served, stained, imbedded etc. I can recommend this method strongly. 

 The collodium is transparent and easily cut, together with the larva. 

 In cases one wants to observe the fixed side, the collodium may 

 easily be dissolved before the preparation comes into the paraffine. 



I preserved free and fixed larvae in the fluid , Kleinenberg used 

 for Lopadorhynchus l ). Of all the various methods this gave by far 

 the best result. The ciliae were e. g. hardly shorter than in the 

 living animal. Complete series of sections were then cut with 

 Jung's microtom and stuck on the glass in the ordinnary way. 



Utrecht, Oct. 26, 1889. 



1) Zeitschr. Wiss. Zool. XLIV, p. 25. 



