73 



(I want to point out emphatically that, up to now, I have 

 exclusively spoken about the normally occurring cases; I will 

 treat the exceptions below.) 



III. Now we come to the question, ivluj in nature a sponge in 

 light — so a green sponge — contains a moderate number of co- 

 lourless algae and a sponge in darkness — so a colourless one — 

 rather a large number of these algae. 



Also here we can make use of an equation for the number 

 of dead (colourless) algae : 



> 

 / + /• + mo = e + g + d 



< 



(The letters stand for the same as in the formulae above ; 

 d means the number of colourless algae disappearing (by solu- 

 tion) in the unit of time from the unit of sponge-volume). 



We know, however, that i = (pag. 51), r = (pag. 53), 

 e= ± (pag. 52), and d = ± (at least before the sponge is full- 

 grown: pag. 57). So we get; 



> 

 mo = g 



< 



of course it is (p. 57), 



eg. for a green sponge in light: mo y g 



A V 

 consequently for a colourless one in darkness : mo > g 



as will show from the data given on pag. 60 — 61 and 53. So we 

 see that in the same time the number of colourless (dead) algae 

 must increase much more in the sponge in darkness than in that 

 in light. 



Lateron I will treat, why the number of colourless algae with 

 structure remains constant during the development of the sponge 

 tissue (chapt. VIII). Why that of the colourless ones without 

 structure increases is mentioned on pag. 56 — 57. 



