200 Biology in America 



study of tlio growth not only of normal but of pathological 

 tissues, such as tumors and cancers. 



When a bii of tissue is removed from a living, or recently 

 killed animal, and placed in a suitable medium (blood plasma 

 is the one mostly emjjloyed) at a pr()i)er temperatun^ it 

 sooner or later, depending on the age of the animal from 

 which it is taken, begins to grow, sending out sheets of cells 

 in all directions into the surrounding medium. After a time 

 however growth ceases, but may recommence if the tissue be 

 removed, washed and transferred to a fresh medium. In 

 this way tissues have been kept alive for more than nine years 

 and carried through nearly two thousand transfers. Carrel 

 has grown chick tissues in this way, which had been kept in 

 cold storage for six days, and even human tissues taken from 

 a cadaver several hours after death may grow. 



But what is death if our tissues, as well as our actions, will 

 live after we are gone? Does the grim specter lie in wait 

 for us in the coils of our intestines, as jMetsehnikolf would 

 have us believe ? Or is it the hardening of our arteries which 

 ushers us into the great unknown? Is death inherent in life, 

 or were the first living things immortal, and death an adapta- 

 tion secondarily acquired for the benefit of the race, although 

 working to the detriment of the individual, as AVeismann has 

 suggested ? 



For an answer to these questions let us turn to the uni- 

 cellular organisms and see what they have to teach us. If 

 a single Paranuecium be put in a fresh infusion of hay in 

 water it soon divides to form two daughter cells, which divide 

 again in their tuin, and so on ; until the infusion is teeming 

 with millions, all offspring of one cell, which is still living 

 in its descendants. For this reason Weismann maintained 

 that the Protozoa were immortal. But after a time repro- 

 duction ceases and the Paramo'cia begin to die off. The cul- 

 ture has passed its climax and begun to retrograde. Finally 

 the Pai'am(ecia disappear entirely, unless fresh material be 

 meantime added to the culture. But if this be done the 

 cells ac(iuire a new lease of life and connnence to multiply 

 again as merrily as ever. 



By using a varied culture of hay, leaves, moss, etc. in 

 rotation and transferring his animals daily to fresh culture. 

 Woodruff has carried a race of ParauKccia through some seven 

 thousand generations extending over a period of twelve years, 

 without any evidence of degeneration. While an exact analy- 

 sis of the different stimuli controlling the Paramo^cia in a 

 hay infusion has not been made, it has been shown ])retty 

 conclusively that waste products materially check their 

 growth, while purity of the culture in this respect stimulates 



