The Coeloni or Body Cavity 287 



into the yolk sac and pushes the walls of the latter before it. 

 The different parts of the brain soon become recognizable and 

 the nose, eyes and ears are also quite apparent, while, upon the 

 sides of the head and neck, the visceral arches and clefts are 

 seen. 



The CcEiyOM or Body Cavity 



On the eighth or ninth day after fertihzation in small animals, 

 perhaps somewhat later in the larger ones, the coelom, or body 

 cavity, is formed as a cleft or rearrangement of the cells of the 

 niesoblast, by which an extensive cavity is formed, radiating 

 outwards from the region of the notocord, to pass beyond the 

 embryo itself and extend outward in the walls of the blastoderm 

 to near the margin of the mesoblastic area. This change serves 

 to divide the mesoblast into two layers, the external of which is 

 closely adherent to the epiblast, the two constituting the somata- 

 pleur, while the inner mesoblastic layer and the endoblast, with 

 which it is intimately related, constitute the splanchnopleur. 



From the epiblast arise finally the epidermis, the hair, nails, 

 hoof, horns, etc., and the cerebro-spinal nervous system. From 

 the mesoblast arise the muscles, bones, connective and other 

 skeletal tissues and the deeper layers of the skin. 



The mesoblast of the splanchnopleur gives origin to the heart 

 and the muscular portions of the digestive, respiratory and urino- 

 genital organs and, in a general way, to the pale, or unstriped, 

 muscle fibers, while the somatopleuric mesoblast forms the 

 striped, or voluntary, muscles. The endoblast of the splanchno- 

 pleur forms the epithelium of the digestive and respiratory tracts. 

 The origin of the various tissues from the three embryonic layers 

 is of great interest in the study of medicine and surgery and 

 numerous functions can only be understood by recalling the em- 

 bryonic origin of certain tissues in the adult. A wound of the 

 epithelium can be repaired by epithelial tissues only. Cells 

 originating from the mesoblast have not the power to repair the 

 cpiblastic injury. 



Ere the blastoderm can proceed far in its development, new 

 provision must be made for its nutrition, which, in mammalia, 

 must be derived from the mother through an intimate relation 

 between her uterine mucosa and special organs of the fetus, 

 the fetal envelops, especially the placenta. 



