332 INVERTEBRATA CHAP. 



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

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

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



Where that region is it is not difficult to determine. If the 

 micromeres of the first quartette be separated from each other by 

 allowing the. embryo to develop in artificial sea-water devoid of 

 calcium, then each micromere will develop into a closed ectodermic 

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

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

 Patella the apical plate is formed in larvae developed from each of 

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

 development of Dentalium a case of specialization, similar to that 

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

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

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

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

 development of Phinocera as compared with that of other Polyclade 

 Platyhelminthes. 



These remarkable experiments of Wilson establish in the most 

 incontrovertible manner the existence of specialized organ-forming 

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

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

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



PELECYPODA Dreissensia 



We must now consider the development of that great group of 

 Mollusca familiarly known as bivalves and scientifically as Pelecy- 

 poda or Lamellibranchiata. The most complete and satisfactory 

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

 by Meisenheimer (1901) on the life-history of Dreissensia polt/morpha. 

 This type we may therefore select for more special study. 



Dreissensia. is a genus fo-und in brackish and fresh water both in 

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

 closely resembles the marine genus Mi/tilus, the common mussel, to 

 which it is regarded by many authorities as nearly allied, and from 

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

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

 posterior opening prolonged into two separate tubular siphons. It 

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

 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 Dreissensia 

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

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

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



