206 Eelation of Nervous System to the Developing Musculature 



The general arrangement of the organs of tlie trunk and tail is well 

 shown in cross sections (Figs. 7 and 8) of the specimen represented in 

 Fig. 3, which was preserved six days after the operation. The al)sence 

 of the dorsal fin and the medullary cord is striking. The notochord, 

 which is normally developed, lies almost immediately helow the epidermis 

 in the trunk (Fig. 7). The greater part of the axial musculature is 

 intact, only the dorsal portion having been cut off. The arrangement of 

 the fibers in the myotomes is normal, except that the individual fibers 

 are often separated from each other by quite large, clear s]-)aces. In 

 the tail the musculature of the two sides arches over the notochord dor- 

 sally (Fig. 8), forming a continuous sheet, horseshoe shaped in section.- 

 Examination of sagittal sections of embryos wdiich had lived from five 

 to seven days after the removal of the cord corroborates the results of 

 the observations upon the living specimens, as'regards the arrangement 

 of the musculature. The division into myotomes is distinct. The prim- 

 ary abdominal muscle is seen as a band of fibers arising from the ventral 

 edge of the myotomes at the base of the tail, skirting past the bud of 

 the hind leg and spreading out anteriorly into a sheet of cells in the 

 abdominal walls. The anterior portion of this muscle is, as is normally 

 the case in this stage, composed of spindle-shaped cells with little or no 

 contractile substance. 



While the above account holds for the best specimens, many cases were 

 observed in which there was much irregularity in the arrangement of 

 the muscle fil)res. Such irregularities are more marked in the imme- 

 diate neighborhood of the scar. They are undoubtedly due to a dis- 

 arrangement of some of the cells at the time of the operation and to 

 uneven healing of the wound. 



The study of the muscular tissue with highly magnifying powers re- 

 veals in the best instances a perfectly normal differentiation of its ele- 

 ments. From this condition there are to be found all gradations down 

 to that shown in some of the poorer specimens in which the degeneration 

 of the elements is marked. In the injured embryos there is a distinct 

 retardation of the differentiation of the muscle fibres, corresponding to 

 the slower development of the organism as a whole. In a specimen 

 killed three days after the removal of the cord there is thus but a small 

 amount of contractile substance laid down in the myotomes and the 

 muscle cells still contain a large amount of yolk. Cross striations of the 

 fibrils may, however, be made out. In a series of sagittal sections of an 

 embryo killed six days after the operation the differentiation of the 

 muscle fibers shows a marked advance. The yolk spheres are almost 



