292 HISTOGENESIS 



mesenchyme (see p. 63), and that derived from a single mesodermal segment con- 

 stitutes a sclerotome. The sclerotomes ultimately are converted into connective 

 tissue, into the vertebrae, and into the basal portion of the cranium. The per- 

 sisting lateral plate of the mesodermal segment becomes a dermo-myotome, from 

 which the voluntary muscle is differentiated and probably the dermis of the in- 

 tegument. 



In the head region cranial to the otocysts no mesodermal segments are formed, 

 but the primitive mesoderm is converted directly into mesenchyme. Mesen- 

 chyme is derived also from the somatic and splanchnic mesoderm and from the 

 primitive streak tissue. From the mesenchyme a number of tissues are developed 

 (see p. 64) . The origin of the blood and primitive blood-vessels and lymphatics 

 has been described; it remains to trace the development of the supporting tissues 

 (connective tissue, cartilage and bone) and of the smooth muscle fibers. 



THE SUPPORTING TISSUES 



The supporting tissues are peculiar in that during their development from 

 the mesenchyme a fibrous, hyaline or calcified matrix is formed which becomes 

 greater in amount than the persisting ceUular elements of the tissue. 



Connective Tissue. — Different views are held as to the differentiation of con- 

 nective-tissue fibers. According to Laguess and Merkel, the fibers arise in an 

 intercellular matrix derived from the cytoplasm of mesenchymal cells. Szily 

 holds that fibers are first formed as processes of epithelial cells and that into this 

 fibrous meshwork mesenchymal cells later migrate. The view generally ac- 

 cepted, that of Flemming, Mall, Spalteholz and Meves, is that the primitive con- 

 nective-tissue fibers are developed as a part of the cell, i. e., are intracellular in origin. 



The mesenchyme is at first compact, the cell nuclei predominating. Soon a 

 syncytium is developed, the cytoplasm increasing in amount and forming an open 

 network. Next the cytoplasm is differentiated into a perinuclear granular 

 endoplasm and an outer distinct hyaline layer of ectoplasm (Fig. 283, A). In the 

 ectoplasm fibrils appear, derived from coarse filaments known as chondrioconta 

 (Meves). 



Reticular Tissue. — Single fibers of reticulin arise in the ectoplasm of the 

 mesenchymal syncytium. The nuclei and endoplasm persist as reticular cells. 

 According to Mall, reticular fibers differ chemically from white connective-tissue 

 fibers 



White Fibrous Connective Tissue. — The differentiation of this tissue may be 



