No. 2.] LIMB-MUSCLES IN AMPHIBIA. 109 
as masses of embryonic cells densely packed with yolk-granules, 
and form projections from the ventral, outer borders of the 
myotomes (Pl. X, Fig. 1). 
Even in their earlier stages the myotome-processes appear as 
distinct diverticula whose cavities are continuations of the 
myocoel. As the myotome-processes elongate and the yolk- 
granules are gradually absorbed, the cells of the rudimentary 
ventral processes become arranged in a single row along the 
walls of the myocoel, as shown in Pl. X, Fig. 2. During the 
ventral growth of the myotome-processes, the cells lose their 
rounded, embryonic appearance and become elongated dorso- 
ventrally in the direction of growth. So pronounced is the 
dorso-ventral elongation of the nuclei of these cells that their 
characteristic shape often serves as a means of identifying the 
cells of the myotome-processes, even when they are closely 
crowded by other structures, as in the region of the anterior 
limbs (Pl. X, Fig. 4). As the myotome-processes elongate, the 
myocoel becomes greatly reduced, and finally wholly disappears, 
leaving double strands of nuclei in place of the thick-walled 
diverticula of the younger stage (Pl. X, Figs. 6, 7). 
While these changes are going on the myotome-processes in 
the fourth and fifth trunk-segments become separated from 
their respective myotomes by the developing pronephros. 
Throughout the length of the body the myotome-processes on 
each side, including those that have lost their connection with 
the myotomes, become united into a thin lateral sheet of muscle 
(‘the primary abdominal muscle”’). The primary abdominal 
muscles taper anteriorly into narrow strands which pass to the 
ventral side of the body, where they become attached to the 
hyoid cartilage. This anterior, ventral portion of the primary 
abdominal muscle becomes the sterno-hyoid muscle. In its mode 
of origin it closely resembles the coraco-hyoid muscle of the 
teleosts. Beyond this stage I have not followed the ventral 
1 Harrison has shown that in the salmon the myotome-processes which are 
very early constricted off from the ventral edges of the anterior myotomes in the 
region of the pectoral fin do not go into the fin as muscle-buds, but unite with 
each other to form the coraco-hyoid muscle. Corning formerly described these 
detached myotome-processes in the pectoral fin-region as muscle-buds. Later he 
confirmed Harrison’s account. 
