338 GENERAL PRINCIPLES 



which later splits to form the body cavity, and again it is gradu- 

 ally formed by cells which sink inward, one by one, from the 

 entoderm and finally arrange themselves around the body 

 cavity. 



720. In general, after the mesoderm and notochord have been 

 formed, the entoderm which remains forms the lining of the 

 digestive tract; that is, the mucous epithelium with all the 

 glandular organs connected with it, salivary, thymus, thyroid, 

 gastric, hepatic, pancreatic and intestinal glands. The mucous 

 epithelium lining the tracheae, bronchi and lungs, is also of 

 entodermal origin. 



721. The mesoderm gives rise to all other parts of the body: 

 the dermis, muscles, all connective tissues, including bones, 

 cartilage, teeth (in part), ligaments, tendons, fascia; heart, 

 blood vessels and blood; lymph, lymph glands, and spleen; the 

 gonads and kidneys with their ducts; the pleura, pericardium, 

 peritoneum and mesenteries. 



722. Indirect Development.— The course pursued by the 

 developing organism is not always the most direct. When 

 the embryo gradually assumes the characters of the adult the 

 development is said to be direct. Often, however, there is a 

 sudden change in direction of development, so that the earlier 

 steps seem to be directed toward a very different goal from that 

 fmally reached; so, for example, in most forms which are fixed, 

 or very sluggish in the adult, there is an active free-swimming 

 larva (Echinoderms, most Molluscs, Worms, Barnacles). This 

 is called a dispersal larva because it seems to distribute the spe- 

 cies partly by its own efforts, but more largely by the currents 

 by which it is carried about. An interesting exception, which 

 proves the rule, is that of the larvae of the fresh-water clam. 

 If this were to follow the rule the larvae would be carried down 

 stream by the currents much farther than the adult would be 

 able to move upward during its entire existence. The result 

 would be that the entire species would soon be carried to the 



