EXPERIMENTAL ANALYSIS 



of protein breakdown products may be a limiting factor in the cell 

 environment, either as nutrients or as specific growth stimuli. Fischer^' 

 has reported his intention to make 'a closer investigation of the mitosis- 

 stimulating effect of higher peptides', and suggests that earlier evidence 

 points to the probability that 'components possessing maximal activity 

 in respect of the induction of cell proliferation' lie within definite limits 

 of molecular weight, in that they are non-dialysable but not precipitated 

 by trichloroacetic acid. Bonus Jensen has found (Fischer^^) that the 

 'presence of large peptides in the medium causes a remarkable increase 

 in the frequency of cell divisions'. Proteins completely broken down to 

 amino acids have an important nutritional function for the tissues 

 (Fischer^^ i"°), but there is no evidence for an effect of any single 

 amino acid, or combination of them, specifically on cell division. 



Chicken embryo extracts 



By far the greatest amount of work on the growth of fibroblasts in tissue 

 culture, from 191 2 onwards (Carrel^^) has been done with chicken 

 embryo extracts as source of growth-promoting factors. Special interest 

 therefore attaches to their mode of action. The activity of chick embryo 

 extracts is not restricted to homologous tissue, and extracts of other em- 

 bryos (and of adult tissues) have been used for cultivating chick tissues. 

 It can therefore be concluded that there is no species-specific growth- 

 promoting factor. Extracts of chick embryos promote growth of cultures 

 of duck fibroblasts (Fischer;^"^ Kiaer^"-), rat tissues (Mottram^^^) and 

 mouse, rat, rabbit, guinea-pig and human tissues (Fischer '^^j. Bovine 

 tissue extracts have been used for chick (Fischer '''2) and human (Gey 

 and Gey^^*) tissue cultures. Chick tissues have been grown in extracts 

 of mouse, guinea-pig and rabbit embryos (Carrel and Ebeling;^''^ 

 Landsteiner and Parker^"^). No significant difference in growth- 

 promoting power has been noted between extracts prepared from whole 

 chick embryos and those from broadly separated regions {e.g., head, 

 eyes, body) of the embryo (Hueper et alii;^^ FowlerI^'), though 

 Fischer'^ found extracts of internal organs less active than those from 

 whole embryos. Gaillard^^^ 10^ has shown that extracts from embryos 

 of different ages may differ in their effects in vitro. Extracts from 

 embryos of successive ages were tested, each at several dilutions, for 

 their growth-promoting effects on tissues from embryos of various ages. 

 Optimal growth of periosteal fibroblasts from lo-day embryo was 

 attained in a 1:8 dilution of lo-day embryo juice; of similar tissue 

 from 15-day embryo in a 1:64 dilution of 15-day embryo juice. 

 Optimal growth in extracts from embryos of the same age as the tissue 

 cultivated was not, however, a general finding — e.g., heart fibroblasts 

 from a 7-day embryo grew best in a 1:16 dilution of 18-day 

 embryo juice (Figure 61). Progressive differentiation, instead of 



173 



