142 TEMPORAL ORGANIZATION IN CELLS 



properties of cells. However /'j- remains thoroughly obscure, and until such a 

 quantity can be observed and measured, equation (92) can have httle meaning. 

 Another environmental parameter commonly used in the investigation of 

 temporal organization in cells, is light. An extremely interesting and complex 

 literature has grown up about the response to different light regimes of a great 

 variety of organisms from unicellulars to bees to man. It is from these studies 

 that most information about frequency demultiplication has been obtained, as 

 discussed in Chapter 6. However, as an experimental tool for investigating the 

 validity of the present theory, light does not seem to offer any clear approach. 

 Cells are extremely sensitive to light and even very brief flashes are sufficient to 

 reset clocks to new regimes (cf. Bruce and Pittendrigh, 1957). For studying 

 entrainment this provides a very useful environmental parameter, but it is 

 difficult to see how it could be put to use in testing the major result of our 

 theory: the existence of different levels of oscillatory excitation in cells whose 

 microscopic state remains unchanged. 



Talandic Properties of Embryonic Systems 



We want to turn now to the question of what significance the present theory 

 might have with respect to the temporal organization of embryonic cells during 

 development. It is clear from classical embryological studies that the timing of 

 biochemical events in differentiating cells is a very significant aspect of em- 

 bryological development, and the phenomenon of competence demonstrates 

 the importance of the right cell state occurring at the right time. There is good 

 evidence that the length of time that a cell or tissue is competent to respond to 

 any particular inductive stimulus, is determined by processes within the cell 

 and is to a considerable extent independent of its environment. Waddington 

 (1934, 1936) was the first to demonstrate that neural competence arises inde- 

 pendently of an endodermal stimulus in the chick, and that in the Amphibia 

 there is an autonomous loss of competence in isolated gastrula ectoderm. 

 Then Holtfreter (1938) showed that the duration of various competences in 

 isolated blastula estoderm of the newt agrees very well with the apparent 

 duration of these states in the normal embryo. The period of competence 

 varies with its nature, whether to form brain, muscle, balancer mesenchyme, 

 etc. The shortest competent period observed by Holtfreter in this material 

 was the capacity to form brain or muscle, the competence lasting 15 h. The 

 longest observed competence was 46 h, this being the capacity to form dis- 

 organized neural or mesenchyme cells, again in the blastula estoderm. 



Another interesting case in which a timing mechanism of some kind appears 

 to be involved, is found in Curtis's (1961) studies on the sorting out of em- 

 bryonic cell types (ectoderm, mesoderm, endoderm) in the formation of 

 primary germ layers in the very early developmental stages of Xenopus laevis. 

 The theory which Curtis advances to account for his observation, is that 

 different cell types reach at different times a state of competence to respond to a 

 particular stimulus, which he believes to be connected with surface shearing 

 forces acting between cells. Once this competent state is reached, the surface 

 properties of the cells are altered so that they become adhesive and stick to- 



