ix MOLLUSCA 309 



powerful cilia. These were obviously the secondary trochoblasts 

 originating from the second quartette ; and the fact that there were 

 two, suggests that normally two cells are contributed to the forma- 

 tion of the prototroch in each quadrant by this quartette. At 

 the other end of the larva were two feebly ciliated cells. These 

 appear to be cells belonging to the third quartette which normally 

 form part of the ventral ciliated groove of the larva, they bear 

 exceedingly fine cilia (Fig. 243, C). 



If one of the residual macromeres 2A, 2B, 2C, or 2D was isolated, 

 i.e. a macromere of the 16-cell stage, a very similar development was 

 obtained. Of course, only cells of the third and fourth quartette were 

 produced, and the formation of a complete larva was very rare. Never- 

 theless in a few cases this did occur. When it did occur the larva 

 had an internal endodermic mass of cells ; but it had no trochoblasts, 

 though two of its cells bore weak cilia. These latter would have 

 formed part of the ventral ciliated band if they had been a portion 

 of a whole larva. 



When micromeres of the second quartette were isolated they 

 produced, like the micromeres of the first quartette, ovoid ecto- 

 dermic vesicles with one or two ciliated cells at one end. These 

 are the one or two secondary trochoblasts that are normally produced 

 from this quartette. The vesicles often have some cells in the 

 interior, and these cells are almost certainly the so-called mesecto- 

 derm or larval mesoderm, i.e. cells which sink into the blastocoele and 

 normally produce larval muscles. Such cells have been described in 

 Polygordius and in the development of other Mollusca, and they will 

 certainly be found to exist in the normal development of Patella 

 when this has been exhaustively analysed (Fig. 243, D). 



Reviewing the experiments which have been described, we see 

 that they prove conclusively that cells from the early stages of 

 Patella, when isolated, give rise to nothing different from what they 

 would have given birth to had they remained part of the egg, and 

 that therefore the cleavage of the egg, from the first, separates 

 definite organ-forming substances. We are dealing in fact with an 

 egg with specialized structure like the Ctenophore egg, and one 

 which is more specialized in this respect than the Nernertine egg, 

 since the product of one of the first two, or one of the first four 

 blastomeres, is not a dwarf larva of half or quarter size, but a 

 monstrous being with only one-half or one-quarter (as the case may 

 be) of the larval structures. Thus, our views as to the affinities 

 of Mollusca with the Ctenophora, which we deduced from the appear- 

 ance of the early larva of Patella, are strengthened by the constitution 

 of the egg as revealed by these experiments. 



PALUDIXA 



It has been mentioned above that the formation of the in- 

 ternal organs has not been fully worked out in Patella ; nor 



