570 ON INCREASE IN COMPLEXITY [pt. iii 



Armadillo produces four identical quadruplets from one egg, and a 

 single Ggg of the parasite Litomastix truncatellus gives rise to about 

 1500 individuals. 



3*6. Self-differentiation and Organiser Phenomena 



The passage of time between fertilisation and hatching can, it has 

 been found, be divided into three periods. These may be summarised 

 thus: (i) division and re-arrangement of pre-existing material and 

 structure (up to the end of the formation of the germinal layers), 

 (2) primary or non-functional differentiation in which the organ 

 rudiments are determined irrevocably and their early differentiation 

 proceeds, and (3) secondary or functional differentiation, in which 

 the inception of active function by the new organs brings about 

 important consequences. "Up to 1910", it has been said, "the prin- 

 cipal achievements of experimental embryology could be summarised 

 thus: first, that nuclear division in early development was not 

 differential, the nuclei of the embryo all being equivalent; second, 

 that most fragments of the germ could, before the onset of gastrula- 

 tion, if of sufficient size, regulate themselves to produce a whole 

 embryo or an approximation to it; third, that in a few cases 

 definite 'organ-forming materials' existed in the fertilised &gg — 

 visibly differentiated regions causally correlated with the develop- 

 ment of certain organs; and fourth, that Roux's doctrine of the 

 struggle of the parts was valid in the later stages." The intermediate 

 part of development was not, in fact, very well understood. But 

 Spemann's discovery in 1918 that the region of the dorsal lip of 

 the blastopore was a differentiator or organisator, as it was called, 

 threw a new light on the middle period. By grafting a piece of it 

 into another embryo (of different species so as to recognise the parts 

 by their colours) he found that it would cause the host tissues between 

 it and the animal pole to form the primary axial organs, i.e. neural 

 plate, notochord and somites. It would initiate, in fact, the self- 

 differentiation process, and would cause cells which previously would 

 have had the potentiality of becoming almost anything according to 

 their position to set out on their path of irreversible differentiation. 

 Then Brachet showed that the organiser in the dorsal lip region 

 exerted its formative effect behind as well as before. If a cut was 

 made so that continuity between the organiser fragment and the 

 posterior region was interrupted, there would be no formation of 



