5G 



MYOTOMES AND SCLEROTOMES 



embryo to the chorion by the short abdominal stalk is accompanied by a certain retardation 

 of the development of the hind end of the embryo. 



Early stages in the development of the muscles and of the 

 connective tissue and blood-vessels : mesenchyme. It will be recollected 

 that the mesodermic segments were traced to a stage in which each shows a central 

 lumen round which the cells are arranged in an epithelial fashion (fig. 82). In 

 some cases the cavity is occupied by branching cells budded off from the ventral 

 wall. In transverse section each segment is oval in shape, and now the lower part 

 of the inner and ventral walls becomes resolved into a mass of loosely arranged 

 cells, wedge-shaped in section, which encroaches on the cavity (sderotome) (figs. 81 

 and 82). These cells, along with those in the cavity of the segment, divide actively and 

 wander inwards, to invest the notochord (fig. 83) and ultimately the neural canal, 

 in a continuous sheet of loose syncytial tissue known as mesenchyme L (fig. 84). 

 It constitutes the blastema out of which the axial connective and skeletal tissues 

 are formed. 



FIG. 81. TBANSVEESE SECTION or THE HUMAN EMBRYO OF 2'4 MM. (see FIG. 77), MORE 

 HIGHLY MAGNIFIED. (T. H. Bryce.) 



ent, entoderm of yolk-sac : the lines indicate the points of the splanchnopleuric layers which will 

 come together to cut off the gut from the cavity of the yolk-sac ; my, outer wall of mesodermic 

 segment ; me, part of its wall which gives rise to the muscle-plate ; sc, sclerotome ; cce, ccelom. The 

 other structures as lettered in fig. 77. The amnion, having been torn, is not completed in this 

 figure. 



While the sclerotomes are becoming differentiated, the cavity of the segment 

 is reduced to a narrow slit bounded by an outer and an inner lamella derived 

 respectively from the outer and inner wall of the segment (fig. 84). The cells of 

 the inner lamella elongate, become arranged longitudinally, and are ultimately 

 (third week) converted into muscle-cells. Hence this lamella is named the 

 muscle-plate (myotome). It is the rudiment of the voluntary musculature of the 

 body. The outer lamella of the segment retains its epithelial arrangement for a 

 time; according to Maurer, it becomes entirely resolved later into a layer of 

 subcutaneous mesenchyme. 



Balfour originally described both inner and outer lamellae as becoming 

 differentiated into muscle, and in some forms this certainly seems to be the case 

 e.g. Lepidosiren (Graham Kerr 2 ) and pig (Bardeen 3 ). As regards the human 

 embryo, Kollmann (1891) described the outer wall as yielding muscle at least in 

 part, but Bardeen and Lewis state 4 that the whole outer lamella becomes 

 muscular tissue. 



1 For definition of this term, see p. 58. 

 5 Johns Hopkins Hospital Keports ix. 



2 Eep. Brit. Assoc. 1902. 

 4 Amer. Journ. of Anat. i. 



