828 



PHYSIOLOGICAL DEVELOPMENT. 



forming the integument with its appendages, and the double 

 layer forming the alimentary canal with its diverticula. We 

 will take first the differentiation which produces the vascular 

 8} T stem. 



Certain forces producing and aiding distribution of liquids in 

 animals, come into play before any vascular system exists ; 

 and continue to further circulation after the development of 

 a vascular system. The first of these is osmotic exchange, 

 acting locally and having an indirect general action ; the 

 second is osmotic distension, acting generally and having an 

 indirect local action ; the third is local variation of pressure 

 which movement of the body throws on the tissues and their 

 contained liquids. A few words are needed in elucidation of 

 each. If in any creature, however simple, different 



changes are going on in parts that are differently conditioned 

 — if, as in a Hydra, one surface is exposed to the surrounding 

 medium while the other surface is exposed to dissolved food ; 

 then between the unlike liquids which the dissimilarly-placed 

 parts contain, osmotic currents must arise ; and a movement 

 of liquid through the intermediate tissue must go on as long 

 as an unlikeness between the liquids is kept up. This primary 

 cause of re- distribution remains one of the causes of re-distri- 

 bution in every more-developed organism : the passage of 

 matters into and out of the capillaries is everywhere thus 

 set up. And obviously in producing these local currents, 

 osmose must also indirectly produce general currents, or aid 

 them if otherwise produced. . Osmose, however, still 



further aids circulation by the liquid pressure which, it esta- 

 blishes throughout the organism. More marked than the 

 contrasts between the liquids in some parts and those in 

 other parts, is the contrast between the whole mass of 

 liquid in the animal and the liquid bathing its surfaces — 

 either the water in which it is immersed, or the water taken 

 into its alimentarjr canal. Its blood and all its juices being 

 denser than water, the result is an osmotic absorption tend- 

 ing ever to distend all its permeable parts — its tissues, 



