GENERAL CONCLUSIONS 321 



yet been shown to possess any such structure that might serve as a 

 basis for developmental pattern. Evidence of the presence in eggs or 

 other reproductive cells of a space lattice related to developmental 

 pattern is at present lacking' {see also Weiss, 1950). According to 

 Seifriz (1935, 1936, 1938), there is, in protoplasm, a continuity of 

 structure consisting of elongated molecules (as in carbohydrates and 

 proteins) to which the polarity and symmetry of organisms can be 

 attributed, Harrison (1936), too, has advanced a molecular hypothesis 

 to account for specific regional localisations: the embryonic develop- 

 mental pattern is to be ascribed to the protein pattern. Because of the 

 bipolar character of protein molecules, Harrison considers that they 

 may become orientated in the cell possibly in relation to the point of 

 attachment in the ovary; and, in relation to the different chemical 

 properties at the poles, two material gradients will be initiated. Local 

 reactions will also take place at points along these gradients and the 

 comphcations of development will ensue. In all these theories, Child 

 sees the need for introducing metabolic factors without which the struc- 

 ture must remain static, if it can indeed come into existence. Molecular 

 orientation, in short, 'must result secondarily from the metabolic 

 differential pattern.' The diffusion reaction theory of morphogenesis 

 advanced by Turing (p. 34) would account for the differentiation of 

 the zygote and for many aspects of the embryonic development without 

 the need for invoking a specific protoplasmic structural organisation. 



Zoological studies indicate that some differential distribution and 

 segregation of materials are present in all eggs (Weiss, 1939). Most 

 animal zygotes show axial differentiation, the reserve materials being 

 aggregated at the basal pole and those needed for meristematic activity 

 at the apical pole. The interior of the egg is typically in a semi-liquid 

 condition. Although the organisation in the animal egg has often been 

 considered to be based on some kind of framework, no such structure 

 can be detected, the network of fibrils seen in cytological material being 

 considered to be artefacts produced by coagulation. In Weiss' view 

 this, however, 'does not preclude the possibility of a dynamic structure, 

 that is, a definite pattern into which the diverse molecular groups would 

 force one another by mutual specific repulsions, attractions and other 

 interactions.' But he admits that this assumption is wholly hypo- 

 thetical. Microscopical studies justify the view that the surface cyto- 

 plasm of eggs has some organisation, but this 'is simple beyond com- 

 parison, when contrasted with the enormous diversity of local characters 

 appearing in the later course of development.' The progressive 

 development of heterogeneity in the germ, and the specific localisation 

 of organ primordia are not determined by the segmentation pattern, the 

 cleavage pattern being a result of the organisation of the egg cytoplasm. 



