1 64 PATTERNS AND PROBLEMS OF DEVELOPMENT 



cardiac development of birds (Fig. 56), is an example. The mammalian 

 alimentary tract, particularly the small intestine, is another. There are 

 several regions of relative dominance along the tract, beginning with the 

 pharynx. These may function separately, or temporary subordination of 

 one or more may occur. In the small intestine gradients of irritability, 

 latent period, muscle tone, rate of rhythmic contraction of isolated pieces, 

 susceptibility, and respiration have been demonstrated, all corresponding 

 in direction, but some temporarily reversible in direction by local stimu- 

 lation at some level below the dominant region at the pylorus There is 

 a remarkable similarity in functional pattern of the mammalian intestine 

 and the ctenophore plate row (see pp. 106, 327).-'^ The ureter is another 

 organ in which there is evidently a very similar pattern of function. The 

 central nervous system shows various evidences of a gradient pattern, 

 as might be expected from its close developmental association with the 

 gradient pattern of early stages. ^^ 



CONCLUSION 



The data of this chapter, though incomplete or fragmentary and often 

 only indicative or suggestive rather than conclusive, constitute, on the 

 whole, a remarkably consistent body of evidence. While complete agree- 

 ment is lacking on various points, the high degree of parallehsm of results 

 obtained on the same material with different methods is sufhciently ob- 

 vious. There is little room for doubt that the methods are not all con- 

 cerned with the same aspects of protoplasmic differentials or gradients, 

 but it does seem evident that they are concerned with different aspects 

 of the same pattern. The spatial patterns of early development are not 

 merely patterns of physical condition, of structure, of concentration of 

 substances, of metabolism, but of all of these — in short, they are spatial 

 patterns of living. If this is true, we shall learn all that they are only as 

 we learn all that living protoplasms are. Whatever they are, metabolism 

 appears to be the effective factor in the progress of development. Without 

 it physical structure and chemical substances would not bring about de- 

 velopment. Moreover, not only metabolism, but metabolism in a spatial 

 pattern, is essential to orderly axiate development. 



Most of the evidence available indicates or suggests that this pattern 

 is primarily quantitative, or at least that differences in rate of living 

 play primarily a more important part in development than differences in 



J* See Alvarez, 1928, and his citations of special papers, 

 •i"* See, e.g., Herrick, 1924; Coghill, 1929. 



