464 ON INCREASE IN SIZE [pt. iii 



The experiments of the Cohn & Murray type were precisely the 

 converse?of those of Carrel & Ebeling who found that the growth- 

 rate of a standard 2-year-old strain of fibroblasts was affected by the 

 age of the hen from which the plasma culture medium was derived. 

 The older the donor the lower was the growth-index, the younger 

 the donor the higher, i.e. the faster the explant grew. Thus the growth- 

 promoting factor must be present both in the cell-protoplasm and 

 in the circulating blood plasma or "milieu interieur". This growth- 

 promoting factor was for long believed to be a hormone or even a 

 vitamine, but it now seems to be much more probably a special 

 collocation of the right nutrient substances, probably protein 

 breakdown-products. 



Following upon the pioneer work of Ross Harrison, it was found 

 by Carrel; Carrel & Burrows; Fischer; Ebeling; and Carrel & Ebeling 

 that tissues would grow indefinitely in vitro on a medium composed 

 of one part adult plasma and one part embryonic tissue juice. 

 It was then soon found that it was the latter constituent which 

 furnished the necessary factors for cell nutrition and multiplication 

 (Carrel & Ebeling and Carrel). From this point there began an 

 extensive series of researches directed towards the identification of 

 the substance responsible for the effects produced by embryonic 

 tissue juice. Whatever it was, it was not species-specific, for duck 

 fibroblasts were found to grow well in chick embryo extract 

 (Fischer), and vice versa (Kaufmann), while rat tissues could be grown 

 in chick embryo extracts (Mottram), and rabbit embryo extracts 

 would stimulate chick tissues (Carrel & Ebeling) or human tissues 

 (Timofeivski & Benevolenskaia) . 



Results which are interesting in this connection are those of Loisel 

 who substituted duck egg-white for the natural egg-white of the hen's 

 egg and found that the normal development of the intact chick embryo 

 up to the 4th day was not interfered with by these conditions.* 



Carrel & Baker examined the fractions obtained from chick 

 embryo juice, and concluded that the growth-promoting factor was 

 associated definitely with the protein part. The precipitated proteins 

 of the embryo juice (acetone, carbon dioxide, alcohols, acetic acid, 

 etc.) never showed a greater growth-promoting power than the 

 original juice, but often nearly as much. Chemical examination 

 showed that the protein in question was a mixture of nucleoprotein 



* Bouges also, by an injection technique, has replaced the yolk by yolk from other 

 breeds or even species, without interfering with normal development. 



