354 EDWIN y. GOODRICH, 



bud from its myotome can still be traced, for the most part, 

 with easC;, but in the first dorsal, which is a little more 

 advanced in development, concentration is more pronounced. 

 Here some of the posterior and anterior buds are seen to be 

 breaking up into irregular masses of cells, and are rapidly 

 losing their connection with, and becoming separated from, 

 the myotomes from which they arose. 



Fig. 3, a reconstructed first dorsal of an embryo 26 mm. 

 long, shows a slightly different case of the same process of 

 concentration. The anterior buds have become separated off 

 in irregular masses, leaving slender stalks, probably nerve 

 rudiments, attached to the myotomes. 



An embryo 28 mm. long (fig. 6) shows the muscle-buds 

 beginning to acquire their definitive structure. At their 

 peripheral ends they are still buds of embryonic epithelial 

 tissue; but towards the base of the fin they are becoming- 

 changed into muscular tissue (indicated by a paler tint in the 

 reconstruction). A more detailed view of these growing 

 radial muscles is given in fig. 19. 



At this stage we can already distinguish twelve well-marked 

 developing radial muscles, corresponding to twelve original 

 buds. That these become gradually converted into twelve 

 adult radial muscles, and were derived from the buds of 

 twelve myotomes, there can be no possible doubt. An ex- 

 amination of numerous intermediate stages proves it. 



A mass of tissue derived from muscle-buds is becoming 

 converted into radial muscle at each end of the series. 

 Whether each of these masses is derived from a single bud 

 or from several it is extremely difficult to determine. As 

 already noticed, the buds at the extreme anterior and posterior 

 ends of the fin become irregular in shape and heaped up close 

 together, so that it is impossible to make certain how many 

 persist to form the adult muscles at these two points. That 

 some few of the buds di.sa])pear altogether in the course of 

 development seems to bo almost certain ; but possibly two, or 

 even three, persist at this stage. 



The above description applies also to the ilevulopment of 



