A. TYLER 369 



homologous antisera. The transformations also occur m response 

 to certain nonspecific environmental (temperature, pH, etc.) 

 changes. It would be of interest to know whether or not they 

 could be produced by specific cell extracts. 



In experiments on frog embryos Rose (1952, 1955) reported 

 specific inhibition (mostly temporary) of the differentiation of 

 blood, brain, and heart by culturing the embryos in the presence 

 of the homologous tissue or extracts thereof. On the other hand, 

 somewhat similar experiments with chick embr\os by Ebert 

 (1955) showed no inhibition of the de\elopment of the spleen in 

 response either to transplantation of spleen from the hatched 

 chick or to injection of homogenates of adult spleen. Some time 

 ago several workers (Murphy, 1916; Danchakoff, 1916, 1918; Wil- 

 lier, 1924) reported that chorioallantoic grafts of adult chicken 

 spleen caused an enlargement of the spleen of the host embryo. 

 Ebert (1951, 1952, 1954) investigated this effect in considerable 

 detail and employed also ingenious experiments witli radioac- 

 tively labeled material. He has shown that the effect is class 

 specific and quantitatively tissue specific. The effect first appears 

 with spleens taken from embryos of about 14 days incubation and 

 increases with age of the donor. Altliough certain splenic antigens 

 are first detected at 14 days of incubation Ebert does not consider 

 his experiments to demonstrate that any one of these is the effec- 

 tive agent. With S''-labeled spleens and kidne\'s he has shown a 

 specific transfer of radioactivity from the graft to the homologous 

 organ, which was evidently not due to transfer and localization 

 of cells, as might have been suspected from the experiments of 

 Weiss and Andres (1952). From tlie quantities transferred and 

 other considerations, Ebert (1955) concluded that the results 

 favor a "building block" rather than a "template" or catalytic 

 mechanism and that the material transferred is of the nature of 

 whole, specific splenic, protein molecules or large fragments 

 thereof. The experiments pertain, then, more particularly to 

 giowth rather than differentiation. 



Grobstein (1955) has recently reviewed his experiments on 

 specific inductive effects between embryonic mammalian tissues 

 separated by filters whose porosity excludes massive cell contacts. 



