ON THE LATERAL MUSCLE IN TELEOSTEI. 319 
2. On the Nature of the Lateral Muscle in Teleostei. By 
Epwarp W. SHANN, B.Sc., Assistant to the Professor 
of Natural History and Lecturer in Comparative 
Hmbryology in the University of St. Andrews *. 
[Received February 2, 1914: Read April 7, 1914. ] 
(Text-figures 1-3.) 
INDEX. Page 
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1. IntRopDUCTORY. 
During the past two years my research hours have been largely 
devoted to a study of the myology of the pectoral girdle and fin 
of fishes, and recently the Teleosteans have more particularly 
occupied my attention. As the musculature of the limbs is 
entirely derived from the so-called lateral muscle, it is desirable, 
in the first instance, accurately to comprehend the nature of the 
lateral muscle. 
That the primitive composition of the lateral muscle, seen only 
in the embryonic condition of living fishes, becomes highly 
modified in the adult Teleostean is beyond doubt; but it is 
ditticult to reconcile the views of various authors, some of whom 
interpret it in terms of two superimposed layers and others in 
terms of one layer. It would be obviously unwise to advance to 
a description of the myology of the limbs while this fundamental 
question is left undecided. ‘To uphold the single-layer theory of 
thé lateral muscle in Teleostei is, then, the primary object of the 
present paper. 
The complex series of cranial muscles is also derived from the 
lateral muscle, but the secondary modifications which they have 
undergone render highly problematical any conclusions based on 
their arrangement. For this reason the cranial muscles will only 
receive passing consideration, The brunt of the discussion 
will be focussed upon that portion of the lateral muscle which 
lies behind the pectoral girdle; and, in particular, upon the 
caudal region, which is generally accepted as the least modified. 
Though not a primary part of my research, the conclusions 
advanced below are the outcome of it; and they are offered as a 
preface to a section of an original morphological study. 
I wish to record my gratitude to the Carnegie Trust, whose 
* Communicated by Prof. W. C. McInrosu, F.R.S., C.M.Z.S. 
