KDC.AR ALLEN". 



Even juvenile mussels are found within the egg membranes. 

 A stringy transparent gel forms or is secreted about the egg 

 membrane enclosing the developing embryos after they reach 

 the marsupia. It is well formed by the stage at which they begin 

 to turn yellow and partly disappears in late metamorphosis. 



Table II. includes observations on twenty typical individuals. 

 More than one hundred are included in the data from which the 

 average time required for the transition between stages is de- 

 termined. 



TABLE II. 



STAGES OF DEVELOPMENT OF YOUNG IN MARSUPIA OF .4. imbecillis.- 



Numbers 13, 14, 17, 19 underwent a complete reproductive 

 cycle in from 22 to 27 days during August. The calculation of 

 cycle duration by averaging parts of cycles from all available 

 data gives an average cycle duration of 3 to 4 weeks. This 

 period then provides time for fertilization, segmentation, em- 

 bryonic development to the glochidial or larval stage, and meta- 

 morphosis of the glochidium into the juvenile mussel. In 

 Anodonta imbecillis it all occurs in the maternal marsupium, with 

 the possible exception of fertilization. Metamorphosis, which in 



* o in the table indicates empty marsupia during the interval between repro- 

 ductive cycles. Numbers refer to stages characterized in the text. 



