56 LILLIE. [Vol. X. 



muscle and the myocytes have been formed at the expense of 

 the larval mesoblast. 



The mere fact of the common origin of the adductor muscle 

 and the myocites proves them to be formed from homologous 

 elements. This fact, of course, includes their histological 

 identity in early phases of development. As both F. Schmidt 

 (No. 31) and Braun (Nos. 6-9) state definitely that no conti- 

 nuity between the adductor muscle of the larva and those of 

 the adult exists, the conclusion that the larval adductor is 

 merely an accumulation of myocytes is unavoidable. Thus 

 another characteristic glochidium structure is brought into 

 direct line with homologous parts elsewhere. 



It seems almost superfluous to add that the distribution of 

 the cells of the larval mesoblast throughout the primary body 

 cavity does not convert the latter into a true coelom. This is, 

 however, what Rabl has affirmed. Study of the post-larval 

 development has shown that the true coelom appears much 

 later, as the pericardium. 



The larval muscle is composed at first of much elongated 

 cells, with very granular cytoplasm and round nuclei (Figs. 75 

 and 'jS), each of which is provided with two distinct nucleoli. 

 Later on the arrangement of the granules becomes very regu- 

 lar ; the nuclei become oval with their long axis in the direction 

 of the length of the cell ; they are then drawn out into a rod- 

 like shape and sometimes take up nearly half the length of the 

 muscle fibre. This condition is reached in the stage of Fig. 

 91. This section, however, does not show the full length of 

 any of the nuclei. In still later stages the muscle cells show 

 a longitudinal fibrillation. Histological differentiation is then 

 complete. I should add, however, that there is either a frag- 

 mentation or great shrinkage of the bacillus-like nuclei. For 

 in the fully formed glochidium the muscle nuclei are quite 

 minute. 



Structures very similar in appearance to the " Strangzellen " 

 are formed from the primary mesoblast ; they are two stout 

 wings of cells which run from the entodermic sac to the walls 

 of the body, Not only are these different from the myocites 

 in origin, but also in their fate. They soon fall into a clump 



