12 BASIC ASSUMPTIONS 



regarded as further evidence of a common protoplasmic origin. 

 At first it was thought that the " high energy " compounds were 

 found only in the form of ATP (adenosine triphosphate). But 

 further studies have since shown the existence of many other " high 

 energy ' nucleoside triphosphates, e.g. guanosine triphosphate, 

 cytidine triphosphate and uridine triphosphate. Recently other 

 1 high energy ' ' compounds have been discovered which contain 

 sulphur, i.e. acetyl coenzyme A. (Lynen 1952, Lipmann 1958). It 

 is possible that further " high energy " compounds will be dis- 

 covered in the future and this greater variety will make it less 

 obvious that all protoplasm uses the same energetic systems. 



Thus on the biochemical side it seems premature to conclude 

 that all protoplasm has a common origin just because many 

 cells show the components of the glycolysis cycle, citric cycle and 

 the " high energy " phosphate compounds. It is likely that the 

 protoplasm of different animals will show the presence of other 

 schemes for the systematic degradation of carbohydrates and then 

 perhaps in time an analysis of these systems will allow us to come 

 to further conclusions about the varied metabolism of protoplasm. 



Morphological evidence. A line of argument developed by 

 morphologists to show the common origin of living cells is the 

 almost universal occurrence of the mitotic and meiotic cycle. Thus 

 Grasse (1952) suggests that such a system indicates the mono- 

 phyletic origin of present-day animals and protozoa. But as 

 Boy den (1953) pointed out, the mitotic cycle is not so fixed or so 

 invariable as people imagine. 



There are variations such as the presence or absence of intra- 

 or extra- nuclear spindles and the presence or absence of centrioles. 

 Thus Amano (1957) suggests that the chromosomes are separated 

 by extending fibres in animal cells though a different mechanism 

 exists in plant cells. On the other hand Swann (1951) suggests 

 that the chromosomes in the Arbacia egg separate because of the 

 contraction of fibres. In fact a perusal of Schrader's book Mitosis 

 (1953) makes it quite clear that one difficulty in finding a single 

 hypothesis to explain the mechanisms of mitosis in all cells is that 

 there are a large number of different mechanisms of mitosis. It 

 also seems that various tissues synthesise their DNA at different 

 stages of the mitotic cycle and that the chromosomes may be 

 duplicated at these various stages (Leuchtenberger 1958). It is 



