548 LOUISE SMITH 



association with the portion of the first epibranchial that breaks 

 down, begins to show sings of degeneracy at stage II; is much 

 reduced in size and very degenerate at stage III, and by stage 

 IV is so far gone as to be almost indistinguishable form a blood- 

 vessel on the ventral surface of the ceratohyoideus internus that 

 lies beneath it (figs. 35, 39 and 46). The latter, which in the 

 larva was small and insignificant, begins at the same time to 

 proliferate new fibers anteriorly and laterally, so that simul- 

 taneously with the flattening out of the ceratohyal, the origin 

 of the muscle migrates from the medial border of that cartilage 

 to its whole anteroventral surface (figs. 30, 34, 37, 38, 39, 44 and 

 45) . The muscle also increases in size posteriorly and gradually 

 begins to curve around the ceratobranchials (figs. 35, 40 and 46) 

 to form the adult pocket. The greatest change of all comes at 

 its posterior end where, simultaneously with the outgrowth of 

 the new portion of the epibranchial, there appear muscle fibers 

 encircling the cartilage (fig. 36). These seem to be a pro- 

 liferation from the end of the ceratohyoideus, as they are per- 

 fectly continuous with it; but it is barely possible that they may 

 be developed from undifferentiated mesoderm cells retained in 

 the connective tissue through larval life, and thus not strictly a 

 part of the ceratohyoideus internus. Whatever their origin, they 

 develop very rapidly, always keeping pace with the growing tip 

 of the adult epibranchial, so that by stage III the spiral muscle 

 is fully formed (figs. 41 and 42) and by stage IV it has even 

 begun to be filled out as it is in the adult (fig. 47) . 



The hyoglossus and suprapeduncularis, the two wholly new 

 adult muscles, are the ones of whose development I am not as 

 yet quite sure. The former appears in stage II as a mass of 

 almost undifferentiated cells, but quite distinguishable from the 

 connective tissue that surrounds them, and in stages III and 

 IV they have assumed the position of, and a slight resemblance 

 to, the radiating fibers of that muscle as they spread out from the 

 anlage of the lingual cartilage. There is no sign of the supra- 

 peduncularis until stage IV, when a few cells, similar to those in 

 the analge of the hyoglossus, appear within the short fold of 

 mucous membrane now formed in the developing tongue-stalk, 

 and undoubtedly represent the anlage of that muscle. 



