174 PRINCIPLES OF EMBRYOLOGY 



transition took place. For instance the American embryologist Lillie (1929) 

 spoke of a process of 'segregation' or 'differential dichotomy' by which 

 the totipotence of the original egg was sorted out into a set of unipoten- 

 cies distributed to the appropriate parts of the embryo. But although the 

 words totipotent and unipotent may be quite convenient additions to the 

 embryological vocabulary, it is a mistake to allow their seductive teclini- 

 cal flavour to conceal the fact that they suggest no explanation of any- 

 thing. Moliere many years ago made fun of the doctors who thought 

 they could explain the sleep-producing action of a drug by attributing to 

 it a 'soporific quality' ; and a 'developmental potency' is a phrase of the 

 same kind. To say that a certain part of an egg has a potency for neural 

 tube formation, for example, means no more than that it has been ob- 

 served in certain circumstances to become neural tube; and any possi- 

 bility of providing a causal explanation of the phenomenon lies not in the 

 invocation of potency, but in analysing the conditions under which such 

 a development occurs. Spemann's service was to discover phenomena 

 which allow one to pass beyond such tautological concepts as potency, 

 and take the first step in identifying the causal interactions involved in 

 development. Naturally the revelation of the first step immediately 

 prompts new questions as to the steps beyond; but in science, as in much 

 else, cest le premier pas qui compte. 



Spemann's success was partly due to a wise choice of experimental 

 material. The newt's egg is large, and lends itself to the easy performance 

 of grafting and cutting experiments. In the early years of the century 

 Spemann cut the egg in half at various stages from fertilisation onwards, 

 and showed that each half might produce a complete embryo when the 

 operation was made at any stage before gastrulation. During gastrulation, 

 the 'totipotence' of the halves was rapidly reduced, and by the end of it 

 each half gave rise to only a half embryo, whatever the plane in which the 

 cut was made. The crucial problem was therefore to discover what hap- 

 pened during gastrulation to 'restrict the potencies' of the parts. 



A clue was present in the fact that although in some experiments both 

 halves of an early stage gave rise to complete embryos, in others one 

 developed completely while the other formed only a mass of cells lacking 

 any sign of the organs of the embryonic axis. We have seen earlier (p. 148) 

 that in fact only those halves containing the grey crescent material develop 

 properly; but in the newt the grey crescent is not clearly to be seen, and 

 the location of the important region was therefore not obvious. Spemann 

 ran it to earth by a series of experiments on the early gastrula. He found 

 that if he separated dorsal and ventral halves, only the dorsal ones devel- 

 oped an embryo; again, if he cut the gastrula in half along the equator, 



