PHYSIOLOGICAL BASIS OF PATTERN 33 



superficial. In the ciliate infusoria, for example, 

 axiation and morphological differentiation are, at least 

 to a very large extent, confined to the ectoplasm, and 

 in the forms which have been examined the ectoplasm 

 alone shows a gradient in susceptibility (Child, 19 140). 

 As regards at least many plant cells the same is also 

 true. In many of the monosiphonous algae with 

 elongated cells the apico-basal susceptibility gradient 

 is very distinct within the limits of a single cell (Child, 

 1916c, e, 1917a, b, 1920a) as well as in the axis as a whole. 

 Modifications with advancing age, cessation of growth, 

 budding, etc., appear very clearly in the changes in 

 susceptibility. 



The symmetry gradients have been demonstrated 

 in various animals as susceptibility gradients. In cer- 

 tain siphonophores, for example, in which the necto- 

 medusa is bilaterally symmetrical in form, the body 

 ectoderm shows a marked bilateral susceptibility gradient 

 In the developmental stages of the sea urchin (Child, 

 1916a) the symmetry gradients have been observed. 

 In certain polychaete annelid larvae the susceptibility to 

 lethal concentrations has been found to decrease from 

 the median ventral region laterally and dorsally, and 

 in various fishes (Hyman) and Amphibia (Bellamy, 

 191 9) it decreases laterally and ventrally from the 

 median dorsal region. 



In some animals the primary gradients persist 

 throughout life, while in others the original gradients 

 may completely disappear during development. In the 

 development of the hydroid, for example, a very distinct 

 gradient appears in the unfertilized egg and cleavage 

 stages, and the planula shows a well-marked apico-basal 



