MUSCLES 



247 



known whether in man this premuscular tissue is derived from the myotomes or 

 arises in situ. In some lower forms there is clear evidence of a growth of 

 muscle-buds from the myotomes into the limb (fig. 303), while in others it seems 

 to be formed by a budding-off of cells individually from the myotomes into the 

 mesenchymatous matrix. While a priori we should expect the striped limb -muscles 

 to be derived from the myotomes in one way or another, there is no decisive proof 



-am.f 



FIG. 303. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF AN 



EMBRYO OF SCYLLIUM. (Balfour.) 



sp.c, spinal cord; sp.g, ganglion of posterior root; ar, anterior root; dn, dorsal; sp.ii, ventral 

 branch of spinal nerve ; nip, muscle-plate ; mp', part of muscle-plate already converted into muscle ; 

 mp.l, part of muscle-plate extending into the limb ; nl, nervus lateralis ; ao, aorta ; ch, notochord ; 

 sy.g, sympathetic ganglion ; ca.v, cardinal vein ; sd, segmental duct ; st, segmental tube ; du, duode- 

 num ; hp.d, junction of hepatic duct with it ; pan, rudiment of pancreas connected with another part of 

 duodenum; umc, opening of umbilical canal (vitelline duct). 



that this is the case in higher forms ; some observers therefore conclude that the pre- 

 muscular tissue is a differentiation in situ (Paterson, Lewis, Bardeen, and others). 

 Ingalls, 1 in a very well-preserved human embryo of 4'9 mm., describes a distinct 

 budding off of cells from the myotomes into the limb-buds. A stream of cells seems 

 first to proceed from the outer plate, as was described by Kollmann, but later the 



i Arch. f. mikr. Aiiat. Ixx. 1907. 



