430 RICHARD EVANS. 



groups in the interior, are to be regarded as one and the same 

 class of cells, while the cells with granular nuclei form another 

 class. Both classes are derived directly from the cells with 

 vesicular nuclei which retain their blastomeric characters, and 

 are therefore capable of giving rise to any tissues of the 

 sponge. 



This view of the larva enables us to give a rational explana- 

 tion of the development of the gemmule of Spongilla into the 

 adult sponge. Zykoff has shown that all the cells of the 

 gemmule of Spongilla are alike. Each gemmule cell possesses 

 a vesicular nucleus, and in all its characters, morphological 

 and physiological, may be compared to a blastomere of the 

 ovum. Hence the gemmule cells are capable of giving rise on 

 the one hand to collar-cells, and on the other to cells with 

 granular nuclei. The same is true of the cells with vesicular 

 nuclei in the larva, and the developmental processes are strictly 

 comparable in the two cases. 



The gemmule, therefore, is an aggregation of these cells 

 brought together in its formation from the neighbouring parts 

 of the adult sponge. Its constituent cells, having retained 

 their blastomeric characters, are capable of giving rise to the 

 whole sponge. From this it follows that the gemmule of 

 Spongilla cannot in any sense whatever be described as a bud. 



Appendix C. 

 The Development and Structure of the Spicules. 



The facts already known of the development of the spicules 

 of the Monaxonida have been so well summarised by Mr. 

 Minchin in his paper (11) published in this Journal at the 

 beginning of the year 1898, that I shall simply refer the 

 reader to that account. 



There are two kinds of spicules in the species, the develop- 

 ment of which has been the subject of this paper, termed 

 respectively " niegascleres " and " microscleres." The former 

 are smooth, either straight or slightly curved, and sharply 



