458 RICHARD EVANS. 



tion found in the Protozoa will serve as a sufficient reason for 

 its full development in the Porifera. 



The second argument that will have to be examined is the 

 segmentation of the ovum. 



The division of the cell produced by the fusion of a male and 

 female cell is a phenomenon which occurs in both Protozoa and 

 Metazoa. In Eudorina, one of the Volvocinese, the egg-cell 

 divides regularly into two, four, eight, sixteen, and sometimes 

 thirty-two cells^ which lie in a kind of jelly and constitute the 

 colony. In Vol vox, again, the fertilised egg-cell divides with a 

 regularity almost unknown among the Metazoa, and develops 

 a colony of numerous individuals. The colony, becoming 

 spherical in shape, might be described as a veritable blastula 

 if it were a young stage of a multicellular animal instead of 

 being an aggregation of unicellular ones, somewhat more 

 closely bound together than Protozoan colonies usually are. 

 Since such regular division of the egg-cell takes place in 

 Protozoa, the strength of the argument based upon the seg- 

 mentation of the egg in sponges appears to be completely lost. 

 Seeing that such division takes place in Protozoa, we are 

 tempted to ask, what difficulty is there in concluding that it 

 was transmitted along one line of evolution to the Porifera and 

 along another to the Metazoa? 



In the third place, the argument based upon the formation 

 of germ layers must be examined. It is perfectly evident that 

 we cannot argue back from the Metazoa and Porifera to the 

 Protozoa when examining the present argument in favour of 

 the Metazoan theory of the nature of sponges, as was done 

 when dealing with the other two arguments. It will be neces- 

 sary, however, to take some of the Protozoa into consideration 

 even here. 



I am kindly informed by Mr. Minchin that the first histo- 

 genetic differentiation among the simplest and most primitive 

 of sponges, i. e. the Ascons, is the formation of a ciliated layer 

 on the one hand, and of "posterior granular cells " on the other. 

 In Clathrina the youngest larva consists of a ciliated layer and 

 from one to four posterior granular cells, while in Leucosolenia 



