GILL DEVELOPMENT IN MYTILUS. 



73 



rence in the gill ; but the order of developmental stages is not 

 the same. In the latest stage (Fig. 8, E) an interlamellar connec- 

 tion is present at a considerable distance from the tip of the fila- 

 ment ; in the next younger stages (Figs. 8, A ; 8, C) the connec- 

 tion is found nearer and nearer to the tip of the filament. Finally, 

 in the earliest stage (Fig. 8, B), there is no distinct connection ; but 

 the distance through which the two limbs of the filament are in 

 union at the tip as compared with the much shorter distance in 

 Figs. 8, A ; 8, C ; or 8, E, shows a potential interlamellar con- 

 nection in an undeveloped condition. The comparison with the 



B 



Fig. 8. Tips of five consecutive filaments of inner gill of Mytilus of 6 mm. 

 length, showing formation of first interlamellar connection. Filaments represented 

 in order of occurrence in gill; order of developmental stages of interlamellar con- 

 nection is B, D, C, A, E. Magnification no. 



continuous connecting membrane of Modiola is evident, and it is 

 hardly going too far to call this a " Modiola stage " in the develop- 

 ment of the filament of Mytilus. 



Perhaps the most interesting stage in the development of the 

 interlamellar connection is shown in Fig. 8, D, which shows 

 clearly the mode of formation of the Mytilus type of connection 

 from its Modiola-like anlage. The filaments shown in Fig. 8 

 were first studied and sketched as total preparations, then sec- 

 tioned. The study of the total preparation indicated that the 

 area bounded by dotted lines in Fig. 8, D, was not an opening 



