REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. I 1 3 



2. They are of exactly the same form and size in the two genera and in the indi- 

 viduals of diverse size and age. 



3. They are always of the same size and shape when they have reached their com- 

 plete development. 



4. Their valves are of a consistency and hardness in no wise related to their size, 

 as should be the case were they the young of Unio and Anodonta. 



5. Their development is not related to any season of the year nor to a certain age 

 of the animal in which they are found; that is to say, one finds in a single locality at 

 the same time individuals containing eggs, others with little bivalves, and some con- 

 taining even the fully developed organisms. 



6. The enormous numbers which are found at one time in an individual are in no 

 wise proportionate to the number of the adults in any locality. 



7. One can not conceive of organs so delicate as the gills being able to serve as a 

 sort of brood pouch, and there is no other example in the animal series of such a con- 

 dition, although these organs are often the seat of animal parasites. 



Jacobson's statement is thus a curious jumble of half truths and of statements 

 which have since been shown to be entirely incorrect. 



The importance attached to the dispute thus raised was so great that the Academy 

 of Sciences at Paris appointed two of its members, De Blainville and Dumeril, a com- 

 mittee with instructions to examine into and report upon the whole matter. This 

 report (De Blainville, 1828) presents an exhaustive review of the early literature and 

 details certain experiments performed by the committee with a view to testing the 

 matter by direct obser\'ation. These experiments, while tending to confirm the earlier 

 views of Leeuwenhoek, were insufficient for the complete overthrow of Rathke's Glo- 

 chidium Theory, for although the report was unequivocal in its conclusion that the 

 observ^ations of all previous authors and the evidence advanced by Rathke himself did 

 not justify the Glochidium Theory, its lack of evidence from original observations 

 rendered it not entirely conclusive. \'iewed in the light of our present knowledge, its 

 skillful and logical arraignment of Rathke's conclusions shows clearly the scant foun- 

 dation upon which the Glochidium Theory rested, but it was not until the work of 

 Carus (1832) that the question was finally set at rest. This author was able, in the 

 brightly colored eggs of Unio Htloralis, to see the passage of the eggs from the ovary to 

 the external gills and their development there to the mature glochidia, and thus to 

 prove beyond any doubt that the innumerable lar\'ae which crowded the outer gills were 

 the young of the mussels in which they were found. 



The paper by von Baer (1830) anticipated some of the points which Carus made 

 the more clear, and from this time on the serious difficulty for students of the embry- 

 ology was found in the failure to secure, either within the gills of the mussel, or upon 

 removal of the embryos to water, any developmental stages beyond the glochidium. 



The period from Carus's paper (1832) to the date of the discovery by Leydig (1866) 

 of glochidia embedded upon the fins of fishes shows little progress toward a more com- 

 plete account of even the embryonic stages. De Quatrefages, who in 1836 described 



