206 EMBRYOLOGY OF THE LOWER VERTEBRATES ^ ch. 



undergoes 

 differentiation 



The paired fins or limbs become muscularized by very similar, 

 segmentally arranged, buds and it is necessary from the outset 

 to bear in mind that this similarity need have no deeper signifi- 

 cance than that the paired fins also necessarily obtain their muscular- 

 ization from the segmentally arranged myotomes. The process 

 as it occurs in the pelvic fin of the shark Spinax is illustrated by 

 Eig. 114 In the 20 mm. embryo (A) the fin rudiment is seen as 

 a longitudinal ridge and a series of myotomes in the neighbour- 

 hood of this ridge are seen each to be forming at its lower edge two 



projecting muscle - buds. 

 These sprout out into 

 the limb rudiment, assume 

 an elongated form (B) and 

 then become separated off 

 from the myotome (C). 

 Each bud now splits into 

 two layers a dorsal and a 

 ventral and each of these 

 histological 

 and be- 

 comes a bundle of muscle- 

 fibres — one of the radial 

 muscles of the fin : so 

 that four radial muscles 

 are derived from each 

 myotome, a dorsal and a 

 ventral from each of the 

 two original buds. Such 

 is the process in its main 

 outlines. 



The existence of a disturbing complication of this simple scheme is 

 indicated by the adult arrangements, in as much as it can be shown that 

 a single motor spinal nerve (i.e. the nerve belonging to a single myo- 

 tome) is related to more than the four radial muscles to which alone we 

 should expect it to be related were the account which has just been 

 given complete. This discrepancy is brought out particularly clearly by 

 physiological experiments. Careful stimulation of a single spinal nerve 

 very commonly causes three consecutive (dorsal or ventral) radial 

 muscles to contract instead of only two, and in some cases apparently 

 a still greater number. This seems clearly to indicate that the end- 

 organs, in other words the muscle-fibres, belonging to a particular 

 motor nerve or myotome are in the adult not strictly confined 

 within the limits of the two pairs of radial muscles corresponding to 

 that motor nerve or myotome. 



To those who believe in the organic continuity of muscle-cell 

 and nerve-fibre from an extremely early stage of development the 

 idea obviously suggests itself that a shifting of some of the 

 constituents from one muscle-bud into its neighbours takes place 



Fig. 113. 



-Muscularization of. median fin in Lepidostens. 

 (After Schmalhausen, 1912.) 



A, 13 mm. ; B, 21 mm. The muscle-buds, and, in the lower 

 figure, the nerves connected with them are shown in black. 



