86 



HISTOLOGY. 



given off, which surrounds the notochord and produces the vertebra^ 

 cartilages and intervertebral discs. It also extends around the medullary 

 tube. This steam of cells is called the sclerotome. The rest of the seg- 

 ment becomes flattened and plate like, by the approximation of its lateral 

 and medial walls. Thus the central cavity is obliterated. Fig. 101, i, 

 shows a cross section of such a segment. Its medial layer is called the 

 muscle plate or myotome. Here the cells multiply rapidly by mitosis and 

 become elongated lengthwise of the embryo. They are called myoblasts 

 and become the striated muscle cells. The lateral layer of the segment, 

 named the cutis plate or dermatome, was supposed to form only mesen- 



chyma which became the deeper part of the 

 skin. It also forms striated muscles, however, 

 and in the pig it is said to be concerned only 

 with muscle formation. The elongated cells 

 of the myotome become separated from one 

 another by mesenchyma, containing blood ves- 

 sels. Thus the myotome is subdivided into 

 layers and groups of cells which shift about 

 in various directions to become the skeletal 

 muscles of the adult. The mesenchyma around 

 them forms fascia and tendon, and connects 

 with the periosteum which is often derived from 

 the sclerotome. In the adult some of the myo- 

 tomes remain quite clearly denned; thus the 

 muscles of each intercostal space are derived 

 from a single mesodermic segment, the ribs 

 having developed between them. In the ab- 

 dominal muscles several segments have fused. 

 The muscles of the limbs are supposed to 

 arise from myoblasts which have migrated into 



them from the myotomes of the adjacent body wall. Apparently they 

 come directly from mesenchyma. All the striated skeletal muscles, how- 

 ever, are believed to come directly or indirectly from the epithelium of the 

 mesodermic segments. 



In cross section the myoblasts are of rounded outline (Fig. 102), 

 bounded by a delicate cell membrane or sarcolemma. This membrane 

 is in close relation with processes from the adjacent mesenchymal cells 

 and it has been said that the well denned sarcolemma of the adult is 

 essentially a product of such cells. The myoblasts consist of granular 

 protoplasm (sarcoplasm) with coarse fibrils near the periphery and nuclei 

 in the central part. In a given cross section the nuclei of many of the 



FIG. 101. THREE MESODERMIC 

 SEGMENTS FROM AMPHIBIAN 

 (SIREDON) EMBRYOS, OP 

 SUCCESSIVELY OLDER STAGES. 

 (Diagrams after Maurer.) 



m., Muscle plate; c., cutis plate; 

 the former is resolved into 

 muscle fibers, m. f., the latter 

 in part into muscle fibers and 

 in part into mesenchyma, mes. 



