284 HISTOGENESIS 



mesodermal' segment constitutes a sclerotome. The sclerotomes ulti- 

 mately are converted into connective tissue, ^vertebrae, and 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, ho mesodermal segments 

 are formed, but the 'primitive mesoderm is converted directly into mesen- 

 chyme. 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. 56). The origin of the blood 

 and primitive blood vessels and lymphatics has been described (Chapter 

 IX); it remains to trace the development of the supporting tissues and 

 muscle fibers. 



THE SUPPORTING TISSUES 



The supporting tissues are peculiar in that a fibrous, hyaline, or 

 calcified matrix is formed during their development from the mesenchyme, 

 and this becomes greater in amount than the persisting cellular elements 

 of the tissue. 



CONNECTIVE TISSUE 



Different views are held as to the differentiation of connective-tissue 

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

 cellular 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 network mesenchymal cells later migrate. The view 

 generally accepted, that of Fleming, Mall, Spalteholz, and Meves, is 

 that the primitive connective-tissue fibers are developed as parts of the 

 cell, that is, 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. 291 A) (Mall, 1902). In the ectoplasm fibrils appear, 

 apparently not mitochondrial in origin (M. R. Lewis, 1918). 



Reticular Tissue. Fine fibers arise in the ectoplasm of the mesenchy- 

 mal syncytium. The nuclei and endoplasm persist as clasping 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: (i) a prefibrous stage, during which 

 the ectoplasm is formed rapidly by the endoplasm of the cells, and fibrils 



