VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 305 
in a muscle derived from another segment ; and if this change 
in innervation were then to be transmitted by inheritance, it 
is claimed that it is fatal to the first-mentioned view (Johnston, 
1906, p. 63). There would, however, seem to still be question 
as to whether this inheritable mutation related to the directive 
impulse assumed to reside in a nerve fibre, or to a proto- 
plasmic strand, or a something else, that pre-existed and 
determined the course of the nerve. The mutation might 
evidently have related to the one or the other. Furthermore, 
there is frequently question, in the cases cited of such a change 
of innervation, as to whether the definitive innervation was not 
in reality primary, being the only innervation that the parti- 
cular fibres under consideration had ever had in ontogeny, 
instead of being secondary in the sense of replacing an earlier 
and normal innervation by the nerve of the segment to which 
the muscle belongs. 
My work, it may here be stated, [in no wa.y attempts to 
solve this vexed and very complicated question. It does 
however raise serious question as to several of the examples 
that have heretofore been cited of the so-called secondary 
innervation of a muscle, and it also quite unexpectedly adds 
a series of instances in which there must be such an innerva- 
tion if existing descriptions of the innervation are correct. 
Before describing my own investigations, limited to a few 
Selachii, it will be well to point out some of the inconsistencies 
and contradictions in earlier descriptions of the development 
and anatomy of the visceral-arch muscles in these fishes. 
Branchial Arches. 
Dohrn (1884, pp. 109-115) says that the myotome of each 
of the branchial arches of selachians, meaning the Plagiostomi, 
becomes flattened “ in the middle,” and is finally there 
separated into two parts, and the descriptions and figures 
both show that this flattening and subsequent separation 
takes place antero-posteriorly along a dorso- ventral line 
passing through the middle of the externo-internal depth of 
the myotome. The myotome is thus here separated into 
