262 THE BIOLOGY OF THE FROG chap. 



hand, if blood be drawn into a vessel whose sides are smeared 

 with oil, it may be kept from clotting for a comparatively 

 long time. Cold greatly checks clotting, the process being 

 delayed almost indefinitely if blood is kept near the point 

 of freezing. If blood be heated to near ioo° C, its power 

 of clotting is destroyed. The formation of clot is de- 

 pendent in some way upon the presence of calcium salts in 

 the plasma ; for if these are precipitated out, clotting may be 

 prevented. The process of clotting is due to the formation 

 of fibrin from a substance called fibrinogen which exists in a 

 state of solution in the plasma. The change is due, like 

 the formation of cheese in milk, to a ferment which trans- 

 forms the soluble compound into an insoluble form. The 

 formation of clot has the function of preventing indefinite 

 bleeding after an injury has been received, the contact with 

 foreign bodies causing the clot to form and thereby check- 

 ing further loss of blood. 



The Lymph. — The lymph is a colorless fluid, devoid of 

 red corpuscles, but furnished with numerous leucocytes. Its 

 plasma coagulates, but not quite so readily as that of blood. 



The Production of New Corpuscles. — The corpuscles of 

 the blood, after functioning for a certain time, die and are 

 replaced by new cells. The process of regeneration of new 

 corpuscles does not take place uniformly throughout the 

 year as in higher animals, but is mainly confined to the 

 spring and early summer. During the late fall, winter, and 

 early spring there is a period of inactivity in the production 

 of new cells. Only after the breeding period, when the frog 

 begins to take food, and store up nutriment in the body 

 is there a rapid regeneration of the blood. The process 

 reaches its maximum in one or two weeks, after which the 

 production of new corpuscles suffers a gradual diminution 

 until fall. 



