THE SUPPORTING TISSUES 285 



The sclerotomes ultimately are converted into connective tissue, into the ver- 

 tebrae, and into the basal portion of the cranium. The persisting lateral plate 

 of the mesodermal segment becomes a dermo-myotome, from which the voluntary 

 muscle is differentiated and probably the corium of the integument. 



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

 formed, but the primitive mesoderm is converted directly into mesenchyme. 

 Mesenchyme 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. 54). The origin of the blood and primitive blood vessels and 

 lymphatics has been described; it remains to trace the development of the sup- 

 porting tissues (connective tissue, fat, cartilage, and bone) and of the muscle 

 fibers. 



THE SUPPORTING TISSUES 



The supporting tissues are pecuhar in that during their development from 

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

 greater in amount than the persisting cellular 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 epitheUal cells and that into this 

 fibrous network mesench)anal ceUs later migrate. The view generally accepted, 

 that of Fleming, Mall, Spalteholz, and Meves, is that the primitive connective 

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



The mesench)rme 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. 291 A) (Mall, 

 Amer. Jour. Anat., vol. 1, 1902). In the ectoplasm fibrils appear, derived from 

 coarse filaments known as chondrioconta (Meves, 1910). 



Reticular Tissue. — Fine fibers arise in the ectoplasm of the mesenchymal 

 S)Ticytium. 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 

 divided into two stages: (1) a prefibrous stage during which the ectoplasm is 

 formed rapidly by the endoplasm of the cells, and fibrils resembling those of 

 reticular tissue appear in the ectoplasm (Fig. 291 ^) ; (2) the anastomosing fibers 

 take the form of parallel bundles and are converted through a chemical change 



