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PRINCIPLES OF EMBRYOLOGY 



on successive phases of cell division and cell expansion but also on defor- 

 mations of the whole structure resulting from the changes in pressure of 

 the body fluid contained in it. No very simple physiological account of 

 the general growth pattern can be expected in such complex cases. 



Even when we are dealing with a simpler case, in which the dominant 

 factor is growth in the ordinary sense of cell multiplication and cell 

 enlargement, our present knowledge does not provide a basis from which 

 the phenomena can be easily understood. The growth pattern seems to 



Figure 13.7 

 Diagrammatic curves illustrating the succession of anatomical systems 

 v/hich are predominant in growth rate in mammals (particularly farm live- 

 stock). Curve I relates to the cranium and shanks, bone and gut-fat, which 

 have a high growth rate at an early stage; the neck, main body musculature 

 and subcutaneous fat grow fastest at a rather later stage (curve 2), and the 

 hind-quarters and intra-muscular fat still later (curve 3). The upper set of 

 curves show the situation when the animal is kept on a high plane of nutri- 

 tion; the phases follow one another rapidly. Under conditions of poor 

 nutrition (lower curves) the succession is more long dravm out. (After 

 Hammond I950-) 



characterise whole regions, which may be made up of a number of anato- 

 mically different elements, and it is surprising how often these elements 

 appear to work together in a harmonious way. Fig. 13.8 shows in profile 

 the skulls of a number of different types of dogs. The differences are pre- 

 sumably genetically determined and it is clear therefore that genes affect 

 the pattern as a whole and not only the individual units comprising it. 

 Moreover it must be remembered that each skull is made up of a number 

 of different bones; to mention only the most striking example, the lower 



