286 



Basic Structure of Vertebrates 



expel the contents of the gland. Hairs and feathers are erected by con- 

 traction of delicate muscles, usually nonstriated. The dilator 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 be- 

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

 wall or endothelium having been established, the outer layers of 

 connective tissue and nonstriated muscle are provided by adjacent 

 mesenchyme. 



The heart develops in the region just behind that where the pha- 

 ryngeal clefts are forming. The right and left hypomeres of the meso- 

 derm push ventralwards, and in the median ventral space between 

 them (Fig. 236) accumulate cells derived from the adjacent hypomeres 

 and, therefore, essentially mesenchymal. These cells arrange themselves 

 to form a very thin layer which becomes the endothelial lining or endo- 

 cardium of the prospective heart. In some cases, at first two endo- 

 thelial tubes are formed, lying side by side, later coalescing into one. 

 The thick muscular layer (myocardium) and the outer layer (epi- 

 cardium) of the wall of the heart, also the pericardium lining the 



Fig. 236. Sections cut transversely through the cardiac region of pig embryos of 

 various ages to show the origin of the heart from paired primordia. (A) 5-somite 

 embryo. (B) 7-somite embryo. (C) 10-somite embryo. (D) 13-somite embryo. (Pro- 

 jection diagrams X 50, from series in the Carnegie Collection.) (Courtesy, Patten. 

 "Embryology of the Pig." Philadelphia, The Blakiston Company.) 



