240 PATTERNS AND PROBLEMS OF DEVELOPMENT 



echinoids present a wide range of variation, with aberrant localizations, 

 deficiencies, excess of skeleton, and development of structures widely dif- 

 ferent from those characteristic of the species under natural conditions. 

 Only a few of these are indicated in the figures. 



The definite and orderly character of the reconstitutional changes in 

 localization and differentiation of parts in exogastrulae represents the 

 realization of potentialities in relation to an axiate pattern, modified pri- 

 marily by differential inhibition and secondarily by differential tolerance, 

 conditioning, or recovery. It is also evident that differential inhibition may 

 completely obliterate axiate pattern, ventrodorsality being obHterated 

 with less extreme inhibition than polarity. With increasing degree of ob- 

 literation there is progressively less evidence of localization and differen- 

 tiation of particular parts, and with complete obliteration the individual 

 remains completely anaxiate in development and without any regional 

 differentiation. Particular features of axiate pattern are not fixedly as- 

 sociated with particular regions of egg or embry^o but may be shifted in 

 position with the experimental alterations of the pattern. At present it 

 seems difficult to account for all the results of experiment otherwise than 

 in terms of a primarily quantitative gradient or differential pattern in- 

 volving metabolism, within which new gradients and specific differences 

 gradually arise. When this pattern is altered differentially by external 

 factors, the region of the embryo or larva in which a particular differentia- 

 tion takes place may be shifted in one direction or another; or if the altera- 

 tion is sufficient, the differentiation does not appear. 



OTHER EXPERIMENTS AND INTERPRETATIONS 



The early experiments of Herbst on the effects of artificial sea water, 

 with certain salts or ions increased in amount, lacking, or replaced by 

 others, were primarily attempts to determine what substances were neces- 

 sary for sea-urchin development.'^ They produced various modifications 

 of development, including exogastrulation, but these were described as 

 effects of particular experimental environments. Many of them are simi- 

 lar to the differential modifications discussed in preceding sections of this 

 chapter, but the possibiHty that development might be altered differen- 

 tially in similar ways by many environmental factors or that secondary^ 

 modifications, opposite in direction to the direct effects of external agents, 

 might occur after a primary inhibition, seems not to have been recognized. 

 The significance of many of the modifications described and figured is, 



'* Herbst, 1892, 1895, 1896^, 1897, 1901a, 1904. 



