14 PRINCIPLES OF EMBRYOLOGY 



another set of gene-activities; later differentiation is the result of these 

 activities. Most embryological work has concentrated on the problem of 

 determination, since it has seemed more important, and perhaps easier, 

 to discover how the genes are brought into activity than to study the 

 detailed course of the processes which they control. The idea of determina- 

 tion has therefore become one of the most fundamental in embryology. 

 Recently, however, interest in the later stages of differentiation has been 

 increasing, with the application of new methods, such as biochemical or 

 immunological techniques for following the way in which specific sub- 

 stances increase in concentration. 



The notion of determination is to some extent a relative one. It is 

 defined in the first place experimentally, in that the part of the egg is said to 

 be determined when we do not know any way of altering its later develop- 

 ment, and of course it is always possible that new experimental methods 

 will succeed where old ones fail. We can thus imagine a part being appar- 

 ently determined in relation to one sort of experiment, but not yet 

 determined in relation to some other. Moreover, there is the question 

 of how specific is the end-result. For instance, a part may be 'determined' 

 as eye, since it will always develop into eye whenever it can develop at all ; 

 but there may still be some possibility of controlling which part of the eye 

 it will form (e.g. retina, tapetum, lens, etc.). Usually, in fact, a tissue 

 gradually becomes more and more precisely determined in a series of 

 steps as its development proceeds. Even when it has become as fully 

 determined as it ever does, it may still have a certain restricted range of 

 possible states which it may assume under different environmental con- 

 ditions. Thus if cells from various organs of the vertebrate body are grown 

 in tissue culture, they often lose many of their obvious visible character- 

 istics and present an apparently 'undifferentiated' appearance (cf.Willmer 

 1935). They tend, in fact, to take on one or other of three basic cell forms, 

 the fibroblastic, the epithelial or the wandering-cell types (Fig. 1.2). But 

 when their powers of differentiation are tested by grafting them back in- 

 to the body or otherwise, it is found that they are actually still as narrowly 

 restricted as they were originally. The various alterations which the cells 

 have undergone have not changed their essential nature, but are merely 

 superficial reactions to different environmental conditions. They are usually 

 knovm as 'modulations' (Weiss 1939). One of the most extreme examples 

 has recently been described by Fell and Mellanby (1953); high vitamin A 

 content in the medium causes chick embryonic skin to differentiate in 

 tissue culture into mucus-secreting, often ciliated, epithelium instead of a 

 squamous keratinising type. If the tissue is transferred back into a normal 

 medium the new cells which develop are of the normal squamous kind. 



