STRCCTDliE OF THE LARVA OF SPONGILLA LACUSTRIS. 457 



They are, as it were, budded off, after disappearance of the 

 nuclear membrane, from the central corpuscle of the vesicular 

 nucleus. The importance of such a difference is evident 

 without further discussion. So remarkable is the difference, 

 that we are almost inclined to state that the process wit- 

 nessed in the preparation of the micronucleus of such 

 Protozoa as Paramoecium presents greater resemblance to 

 what occurs in Metazoa than does that occurring in sponges. 

 In the Protozoon just mentioned the micronucleus divides a 

 number of times before conjugation is brought about, and the 

 interchange of micronuclear substance takes place. Some of 

 the products of division in Paramoecium degenerate and come 

 to nothing, much in the same way as the polar bodies do in 

 the sponges ; but the important point is that the micronucleus 

 divides always by mitosis. There is, therefore, greater simi- 

 larity, as regards the point in question, between what may be 

 termed the maturation of the micronucleus in the Ciliata, and 

 that of the nucleus of the egg-cell in Metazoa, than there is 

 between the maturation of the latter and that of the egg-cell 

 in sponges. It seems, therefore, that the Metazoan theory 

 derives no support from the maturation of the ovum, and the 

 facts appear to cut both ways. 



Again, we find among the Protozoa, e. g. some of the Volvo- 

 cinese, both male and female cells. The male cell in Volvox 

 and Eu dor in a is small, active, and motile, being comparable 

 to the spermatozoa of the Metazoa; while the female cell is 

 large, inert, and sought for by the male cell in the same way as 

 is generally the rule among the Metazoa. I do not mean to 

 assert that the fusion of the male and female cells in Volvox is 

 comparable in all its details with what occurs in Metazoa, but 

 the very existence of such a process does away with any difficulty 

 — based on the presence of sexual reproduction in the sponges — 

 of adopting the view of the independent origin of the Porifera 

 from the Protozoa. Since these processes occur in the Protozoa, 

 their transmission during the phylogenetic evolution of the 

 sponges from that group does not after all appear so im- 

 probable. In short, the incipient methods of sexual reproduc- 



