452 PRINCIPLES OF EMBRYOLOGY 



be discussed; that is the fact that they are strongly polarised. It was 

 recognised by Spemann in the early experiments with organisers that if 

 tissue from the blastopore region is grafted into another site in an embryo 

 it tends to go on invaginating in its own original direction, although it 

 very often becomes swung round into conformity with the gastrulation 

 movements of the host. Particularly striking examples of the tendency of 

 a graft to retain its own polarity and direction of invagination can be 

 seen in anurans, such as Discoglossus, in which development is very rapid 

 and the anterior-posterior elongation and lateral narrowing of the blasto- 

 pore material particularly well marked (Waddington, 1941). It is still 

 rather unclear whether this polarity is a property of pieces of tissue, i.e. 

 of groups of cells in which, perhaps there is a gradient in the intensity of 

 the cell-membrane forces, or whether it is inherent in the individual cells 

 of which the tissue is composed. Hokfreter (1947) has given a somewhat 

 diagrammatic drawing of the recently invaginated mesoderm, in which a 

 polar structure of the individual cells is indicated, and he has also shown 

 experimentally that isolated cells do develop a polarity of their own. 

 However, if this cell polarity exists before gastrulation it is certainly by 

 no means irrevocably fixed, since small parts of the presumptive meso- 

 derm of the newt, cut out and replaced with the anterior-posterior axis 

 reversed, may in some cases invaginate in perfect conformity with their 

 surroundings and show no signs of reversed cellular polarity (cf p. 458). 



5. Measurement of the forces and energy involved in morphogenesis 



During morphogenetic changes regions of tissue are shifted about 

 bodily in space; that is to say, work is done, and energy must be expended. 

 Many attempts have been made to divide the energy used by an embryo 

 into a fraction required for simple maintenance of the living system, and 

 another fraction devoted to the performance of this morphogenetic 

 work. In practice it has been found extremely difficult to do this; the 

 earlier work on the subject has been fully discussed by Needham (1931). 

 It has indeed been difficult even to demonstrate the existence of a 

 morphogenetic energy fraction, let alone to measure it. One of the most 

 successful attempts to do so has been that of Tyler, summarised in his 

 review of 1942. He compared the rates of development and the oxygen 

 consumption of embryos derived from whole echinoderm eggs or from 

 separated first blastomeres (half embryos) or from fused eggs (giant 

 embryos). He found that the rate of respiration was nearly the same in all, 

 but that half embryos took a longer, and giant ones a shorter, time to 

 reach a given stage than did normals. Thus by the time they reach a given 

 stage, the half embryos have consumed more and the giants less energy 



