120 H. V. NEAL 



hypothesis requires. On the other hand, there is much evidence 

 of the distribution of motor nerves — such as the abducens — 

 into metameres other than those in which they have their nid- 

 ulus. This fact suggests a second hypothesis of the origin of 

 the relations of the abducens to the posterior rectus muscle. 



According to the second hypothesis, the abducens has attained 

 its present relations by a process of substitution or nerve piracy; 

 that is to say, a post-otic nerve has, in the course of phylogeny, 

 usurped the area of distribution of a pre-otic one. The clue to 

 the process by which this substitution has taken place is afforded 

 by the anterior post-otic nerves in Petromyzon. 



Ontogenetically, the two anterior post-otic somites of Petro- 

 myzon divide into median and lateral divisions, of which the 

 former lie median to the otic capsule and adjacent to the noto- 

 chord, while the latter lie lateral to the otic capsule and to the 

 large ganglia of the ninth and tenth cranial nerves (figs. 78 and 

 79) . The lateral portions of the myotomic divisions divide (com- 

 pletely, in the case of the first post-otic myotome) into a dorsal 

 division {my. 4 d.L), above the otic capsule, and a ventral divi- 

 sion, below the ear, the division occurring along the line of lat- 

 eral line sense organs. The relation between the series of lateral 

 line sense organs — including the otic capsule — and the line of 

 cleavage of the myotomic divisions seems more than merely 

 topographic, and there is little reason to doubt that the devel- 

 opment of the ear and the sense organs and their related gan- 

 glia has been one of the conditions — if not the essential condi- 

 tion — of the splitting of the myotomes. That is to say, the 

 splitting of the myotomes may reasonably be regarded as an 

 adaptation to the conditions brought about by the enlargement 

 of the sense organs and cranial ganglia. The median division 

 of the anterior post-otic myotomes develops embryonic muscle 

 fibers, which degenerate and disappear in relatively early em- 

 bryonic stages, for they are entirely absent in a 50 mm. embryo. 

 With them disappear, it may be inferred, the associated somatic 

 motor nerves; although, in spite of much pains, I have been 

 unable to demonstrate the existence of embryonic nerves asso- 

 ciated with these myotomic divisions. The reason for this fail- 



