434 



EXPLANTING AND CULTIVATING 



Spratt (1947) sets up a group of criteria for the adequacy of the culture medium used in 

 support of development, as follows: 



1. Regression of the primitive streak, formation and elongation of the notochord. 



2. Formation and closing of the neural folds. 



3. Development of the brain and spinal cord. 



4. Formation of somites. 



5. Development of optic vesicles and otocysts. 



6. Morphogenesis and pulsation of the heart. 



7. Formation and extension of a "tail" from the cut edge of the anterior piece. 

 He states: "A medium which meets all of these requirements is considered adequate. " 



From the works of Spratt it seems evident that the early blastoderm does not have ade- 

 quate endogenous food supply (as does the fish embryo) but depends upon some exogenous 

 source for nutrition. The best medium proved to be the entire egg (yolk and albumen) 

 extracted and added to a saline-agar base. The albumen seems not only to be bacterio- 

 lytic but also seems to help maintain the proper pH of the blastodermic environment. A 

 "complete" synthetic medium was finally devised, containing saline-agar, glucose, 

 amino acids, vitamins, etc. on which explants underwent essentially normal morphogen- 

 esis, differentiation, and some growth. However, Spratt subsequently found that the 

 glucose component was the only absolutely essential exogenous source of energy for 

 morphogenesis and differentiation of the explanted early blastoderm. These develop- 

 mental processes occurred in the almost total absence of growth. Mannose, fructose, 

 and maltose were apparently just as efficient as was glucose. 



In a study by Rawles (1936) followed by Rudnick (1938) it has been possible, by explanta- 

 tion and chorio-allantoic grafting, to map the organ forming areas of the early chick 

 blastoderm. Rawles found that the developmental potencies within each area diminish 

 peripherally from the center, and that there is superior developmental capacity of the 

 left side of the blastoderm over the right. 



07MM 



0.3M.M. 



REGIONS OF ORGAN ANLAGE 

 IN EARLY CHICK BLASTODERM oOMM 



D 

 03MM 



TMs chart Is made from the combined 

 data of Wlllier and Rawles 



ECTOOOM 

 MESODERM 



