MUSEUM OF COMPARATIVE ZOOLOGY. 101 
embryo of Pristiurus, each myotome gives off at its ventral margin two 
small buds, one from its anterior and the other from its posterior edge. 
These buds, after losing their connection with the myotome, lengthen 
in a dorso-ventral direction, and then divide parallel to the surface of 
the body, thus forming four secondary ceil-masses for each myotome, — 
two dorsal, or outer, and two ventral, or inner. These masses of indif- 
ferent myotomic elements pass into the limb-bud, and ultimately assume 
positions which make them here also respectively dorsal and ventral, and 
constitute the source of the entire musculature of the limb. Dohrn was, 
however, unable to determine the exact number of myotomes which thus 
contribute elements to the pectoral limb-bud, nor could he ascertain 
which myotome is the most anterior, or which the most posterior, to 
give off buds to the pectoral fin. 
Ziegler (’88, pp. 388-390) has briefly described the source of the 
mesoderm of the pectoral fins in Elasmobranchs. In stages 7 and J 
(of Balfour, ’78) there is produced upon the somatopleure, chiefly by 
cell proliferation, a layer of formative tissue, which extends caudad 
through the pectoral region. This process is accompanied by an up- 
heaval of the ectoderm, and constitutes the beginning of the formation 
of the fore limb. Ziegler believes that this formative tissue (Bildungs- 
gewebe) of the pectoral is continuous with the sclerotome, passing over 
between the segmental tubules. He has also pointed out that the 
dorsal ends of the myotomes at first give off to the dorsal fin formative 
tissue as single-cell strands, and later the myotome buds observed by 
Dohrn and Paul Mayer (’86). It is to be inferred, in the absence of 
any statement upon this point, that similar cell strands have not been 
observed on the ventral sides of the myotomes passing to the pectoral 
fins. 
Beard (’89, p. 114) has observed the contribution of muscle-buds from 
the myotomes in Lepidosteus osseus. He says: ‘ The somites become 
much elongated, and their ends are constricted off as buds to form the 
musculature of the paired and unpaired limbs. Each end of each somite 
constricts off a single bud, which only divides into two at a stage later 
than that of the twenty-first day. The posterior pair of fins begin to 
develop only towards the end of the third week, and the muscle-buds 
of the unpaired fins remain in an embryonic condition beyond this 
period.” 
Van Bemmelen (’89, p. 240) has studied the limb-buds of the Lizard, 
and has pointed out the presence of myotome buds. In a stage in which 
five gill-pouches are developed, the myotomes back of the auditory vesi- 
