332 INVEETEBEATA chai 



lobe does not contain the specific organ-forming material for the apica 

 plate ; in the interval between the formation of the first and seconc 

 polar lobes it has been distributed to a different region of the egg. 



Where that region is it is not difficult to determine. If th( 

 micromeres of the first quartette be separated from each other b] 

 allowing the embryo to develop in artificial sea -water devoid o 

 calcium, then each micromere will develop into a closed ectodermi( 

 vesicle ; but only the micromere Id develops an apical plate, and thi 

 apical "stuff" is therefore transferred to this micromere. Now, ii 

 Patella the apical plate is ibrmed in larvae developed from each o 

 the four micromeres of the first quartette ; we have therefore in th( 

 development of Dentalium a case of specialization, similar to thai 

 which we often meet with in eggs with spiral cleavage, in whiol 

 one member of a quartette does the work normally undertaken by al 

 the sisters in other species. A case of this kind was met with ir 

 the first case of spiral cleavage which was studied, namely, in th( 

 development of Planoeera as compared with that of other Polyclad( 

 Platyhelminthes. 



These remarkable experiments of Wilson establish in the mosi 

 incontrovertible manner the existence of specialized organ-forming 

 substances in the egg of Dentalium. It is but fair to add that th( 

 first experiments of this kind were made by Crampton (1896) on th( 

 egg of the Gastropod Ilyanassa, where a similar polar lobe is found. 



PELBCYPODA — Dreissenda 



We must now consider the development of that great group o: 

 MoUusca familiarly known as bivalves and scientifically as Pelecy- 

 poda or Lamellibrancliiata. * The most complete and satisfactorj 

 study of the development of any form belonging to this group is thai 

 by Meisenheimer (1901) on the life-history of Dreissensia polymorpha 

 This type we may therefore select for more special study. 



JDreissensia is a genus found in brackish and fresh water both ir 

 England and on the continents of Europe and America. In form i1 

 closely reseuibles the marine genus Mytilus, the common mussel, tc 

 which it is regarded by many authorities as nearly allied, andfron 

 which it differs in having the two mantle lobes firmly united for i 

 part of their length in the mid-ventral line, and in having th( 

 posterior opening prolonged into two separate tubular siphons. Il 

 is interesting from the fact that, though a fresh-water species, it retain) 

 a long larval development of very primitive facies, whereas most fresh 

 water species have a shortened,modified,and mainly embryonic develop 

 ment. Dreissensia is clearly a recent immigrant into fresh water. 



Meisenheimer obtained his material from one of the small fresh 

 water lakes of Germany (the Ploner See). The eggs of Breissensu 

 polymorpha are laid in June, and are cast forth from the mother ii 

 masses, bound together with a slight amount of slime which is easib 

 washed away. The eggs have no chorion of any kind, and hence ari 



