Io6 COMPARATIVE ANATOMY 



between "dermal" and "membrane" bone. "Dermal" seems the more 

 significant descriptive term. 



Bone resulting from replacement of cartilage is called cartilage bone. 

 It is sometimes called primary bone because its cartilage precedes dermal 

 ossification whose product, therefore, may be called secondary bone. 



Muscle. 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 urino- 

 genital ducts. The outer surfaces of these primary walls are always 

 adjacent to regions occupied by mesenchyme. The unstriated muscle 

 fibers which constitute the muscle layers of the walls of these hollow organs 

 are differentiated from cells of the adjacent mesenchyme. In such organs 

 as the stomach and uterus the muscle layers become thick and strong. 

 They are relatively thin in a lung, air bladder or urinary bladder. The 

 walls of the larger blood vessels in all parts of the body contain unstriated 

 muscle whose fibers lie in planes transverse to the axis of the vessel so that 

 their contraction decreases the diameter of the vessel. Unstriated muscle 

 occurs in the walls of some integumentary glands, serving to expel the 

 contents of the gland. Hairs and feathers are erected by contraction 

 of delicate muscles attached to the follicles in which the organs are inserted. 

 These muscles are unstriated or possibly sometimes striated in case of 

 feathers. Most unstriated muscle of integumentary organs is derived 

 from mesenchyme. The changes in diameter of the pupil of the eye are 

 effected by unstriated muscle in the iris. The dilator fibers in the human 

 iris, however, are apparently of ectodermal origin. Mesenchymatous 

 unstriated muscle, wherever situated, is innervated by fibers from the 

 autonomic system. 



Circulatory Organs. The general statement that the circulatory 

 vessels are derived from mesenchyme is probably admissible although 

 some vessels seem to arise fairly directly from the mesoderm. There is 

 apparently much diversity in the manner of origin of the vessels. They 

 may arise as soHd cords of cells, later becoming hollow, or they may be 

 hollow from the beginning. The essential wall or endotheliiun having 

 been established, the outer layers of connective tissue and unstriated 

 muscle are provided by adjacent mesenchyme. The heart develops as a 

 large blood vessel. In the region just behind that where the pharyngeal 

 clefts are forming, the right and left hypomeres of the mesoderm push 

 ventralwards and toward one another just as they do in a more posterior 

 region of the body. But instead of meeting to form a median ventral 

 mesentery, they bulge away from one another (Fig. 79) so as to give 

 rise to a median ventral space between them. In this space accumulate 



