76 



CHORDATE ANATOMY 



tissue (Fig. 69). Most of the bones which develop in this manner are 

 derived from the embryonic mesenchyme of that same general superficial 

 layer which otherwise gives rise to the dermis of the skin. They are 

 accordingly called dermal bones. Bone resulting from replacement of 

 cartilage is called cartilage bone. 



Mesenchyme is the source of nearly all unstriated or "smooth" muscle, 

 whether in the walls of viscera or in the body-wall. Most visceral organs 

 are hollow. In their early embryonic stages their primary and essential 

 walls are either endoderm as in the case of the digestive tube, lung or 

 urinary bladder; or mesoderm as in the urinogenital ducts. The outer 

 surfaces of these primary walls are always adjacent to regions occupied by 

 mesenchvme. The unstriated muscle fibers of these organs are differen- 



FiG. 70. — Diagrammatic transverse sections of developing heart. In A the descend- 

 ing right and left mesodermal hypomeres have nearly met; mesenchyme cells appear 

 between them. In B the layers have met ventrally forming the ventral mesocardium; 

 the enclosed mesenchyme has formed the endocardium. In C the layers have met 

 dorsally to form a dorsal mesocardium; meanwhile the ventral mesocardium has dis- 

 appeared and the right and left coelomic spaces have become the pericardial cavity. 

 c, coelom; ec, ectoderm; en, endoderm; end, endocardium; m, ventral wall of hypomere; 

 p, pericardial cavity; v, mesenchyme cells. (From Kingsley, "Comparative Anatomy 

 of Vertebrates.") 



tiated from cells of the adjacent mesenchyme. Unstriated muscle fibers 

 occur in the walls of larger blood-vessels and of some integumentary 

 glands where they serve to expel the contents of the gland. Hairs and 

 feathers are erected by contraction of delicate muscles, usually unstriated. 

 The dilators fibers in the iris of the human eye, however, are apparently 

 of ectodermal origin. 



The statement that blood-vessels are derived from mesenchyme is 

 probably admissible although some vessels seem to arise fairly directly 

 from the mesoderm. They may arise as solid cords of cells, later becoming 

 hollow, or may be hollow from the beginning. The essential wall or 

 endothelium having been established, the outer layers of connective 

 tissue and unstriated muscle are provided by adjacent mesenchyme. 



The heart develops in the region just behind that where the pharyngeal 

 clefts are forming. The right and left hypomeres of the mesoderm push 

 ventralwards and in the median ventral space between them (Fig. 70) 

 accumulate cells derived from the adjacent hypomeres, therefore essen- 

 tially mesenchymal. These cells arrange themselves to form a very thin 

 layer which becomes the endothelial lining or endocardium of the prospec- 



