84 



which will allow little else than the eggs to pass through 

 its meshes. The spermaries of the ripe males are treated 

 in the same way in another sancer, after which the male 

 cells mixed with sea- water may be ponred into the dish 

 containing the eggs. In two or three hours after the 

 water containing the male cells has been poured over the 

 eggs, development will have begun. The water should 

 now be removed from time to time by pouring it out of 

 the dish or tumbler, taking care not to stir up the eggs 

 which have settled to the bottom. Soon, however, the 

 embryos will begin to swim, when it will be necessary to 

 remove the water in another way, or with the siphon. 



The changes which occur in the eggs during develop- 

 ment, succeed each other with considerable rapidity, 

 and, as in the development of the oyster, there are very 

 well-marked periods of active change of form, Avhich 

 alternate with periods of repose, while there is a bilateral 

 symmetry which is just as well marked as in the latter 

 and Unto. A power of 250 diameters enlarges the eggs 

 satisfactorily ; shows the freshly impregnated eggs as 

 spheres measuring i inch in apparent diameter, a proof 

 that the actual diameter of the eggs is very nearly l-500th 

 of an inch. The eggs are not spherical when first re- 

 moved from the ovary, but are usually irregular in form 

 with a more or less well pronounced conical extremity, 

 by which they were attached to the walls of the ovarian 

 follicles. The other end is more or less irregularly 

 rounded. A nucleus is plainly visible during this irreg- 

 ular stage, but no well defined egg-membrane is apparent. 



The early stages which I was enabled to figure may 

 not entirely cover the different phases through which the 

 embryos xDass at that time, but in view of the circumstance 

 that there is but little known of the development of La- 

 mellibranchiate or bivalve mollusks, and that little relates 

 to but few forms, I feel justified in offering the results 

 obtained from a study of the embryonic development of 

 the Clam, which has not been studied hitherto. Though 



