158 SEPARATION OF GROWTH AND DIFFERENTIATION 



DRAWINGS OF GBOWTH- INHIBITED EMBRYOS 



DISCUSSION: 



Whltaker and Berg (19't'+) first suggested this method of separating the processes of 

 determlnati-on, growth, and differentiation, using the Fucus egg. Recently (I9U5) Holt- 

 freter has made a similar study on the amphibia with very instructive results. He says: 

 "Embryonic development is brought about by the integrated cooperation of various chains of 

 biological processes, such as cell multiplication, morphogenetic movements, histological 

 differentiation, differential changes of size and form of the embryo, etc., each of which 

 has been given ample attention in analytical research while information on the relation- 

 ship of these processes with each other is scarce and has been the by-product rather than 

 the aspired aim of experimental work." He says, further, that "Instances have been re- 

 corded where cytological differentiation occurred without cleavage, growth without differ- 

 entiation, tissue formation without morphogenetic movements, metabolism without growth or 

 differentiation." 



The agar medium provides an isotonic but restrictive environment where there may be 

 relatively free exchange of respiratory gases but which prevents expansive growth or the 

 acquisition of body cavities (neurocoel, archenteron, coelom) . Such pressure as is ex- 

 erted is due to the forces of expansive growth of the embryo, for it is first exactly 

 fitted into a relatively fluid agar medium. Such uniform restriction does not impede the 

 formation of neural folds, or of other invaginations such as the stomodeum, proctodeum, 

 olfactory pits, etc. The total volume of the egg (or embryo) remains almost static. The 

 evaglnationa, such as the optic vesicles, gills, balancers, and limb-buda are apt to be 

 restricted. Intra-cellular imbibition are limited and concern particularly the mesenchyme. 

 It is not yet known whether mitosis is in any way inhibited, but certainly morphogenetic 

 movements are not. While the general shape and size (growth factors) of the embryo re- 

 mains much as they were at the time of embedding, there is not present a parallel restric- 

 tion of differentiation. 



