82 PROTOPLASM 



daughter cells are nearly separated, but a protoplasmic thread 

 still connects them; at 4:43, the two cells, instead of fully separat- 

 ing, as they normally should, come together again; and at 5:25, 

 one single cell remains with both of the daughter nuclei. 



Of great significance to the whole field of physiology is the 

 problem of the chemical nature of growth-stimulating sub- 

 stances. Important work dealing with this problem has been 

 done by Carrel and his coworkers. Lewis showed that cells 

 will live, migrate, and divide in a pure salt (sodium chloride) 

 solution. They do better in a "balanced" salt solution and 

 apparently still better in a solution containing sugar (glucose), 

 but their most prolific multiplication occurs when an organic 

 substance of greater chemical complexity is present. Carrel 

 discovered that cells would live indefinitely in embryonic tissue 

 juice. He, Ebeling, and Baker set out to ascertain the chemical 

 nature of the substances in embryo extract that are responsible 

 for continued growth. One would suspect these to be protein 

 in nature, and this was found to be true. Substituting amino 

 acids was not successful, even though these hydration products 

 of proteins serve well as substitutes in the nutrition of animals. 

 The proteoses, which are more complex products of the proteins, 

 proved to be suitable for sustained tissue growth. It seems, 

 therefore, that the extraordinary growth-producing powers 

 of embryo extract are due to certain peptic proteins. While 

 the proteoses apparently play the chief role, the complete diet 

 must also contain nucleic acid (an important constituent of 

 chromosomes), glycocoU (the simplest of the amino acids), 

 glutathione (a dipeptide, concerned in the respiration of cells), 

 and oxyhemoglobin. The addition of each of these, succes- 

 sively, to an artificial culture medium, in which no embryo 

 juice is used, increases the rate of growth. Embryo juice 

 either contains all of these, or they can be made by the growing 

 cells from the proteins of the embryo juice. 



Much of the work on nutrition has been on sarcomatous 

 (cancer) fibroblasts. It appears that the requirement of cancer 

 cells differs slightly from that of normal ones. Normal cells use 

 artificially supplied proteins only for a short period of growth, 

 while sarcomatous fibroblasts multiply for a long time in them. 



Vogelaar has developed a nutrient medium with Ringer 

 solution, phosphate, glucose, and peptone as the base, to which 



