DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 241 



therefore, uncertain; but it is evident that many of them are differential 

 inhibitions. It is possible, however, that in some cases a modification re- 

 sulting from absence of a particular constituent of sea water may be 

 more or less specific in character, but to prove this requires extensive ex- 

 periment — it must not be taken for granted. Herbst was concerned with 

 other problems than those of developmental pattern, but many of his 

 data point to the same conclusions as those of this chapter. 



The attempts to interpret differential modifications of sea-urchin de- 

 velopment, including exogastrulation, in terms of a single apicobasal 

 gradient are, in the light of more recent work, applicable only to the 

 earlier stages preceding gastrulation and to the ectoderm of later stages.^'' 

 The change in physiological condition of prospective entoderm and mes- 

 enchyme was not known at the time of these papers; and, though evi- 

 dences of it were observed in some cases, their significance was not recog- 

 nized, nor was dissociation of prospective entoderm actually observed. 

 As regards the ectoderm, however, the interpretation suggested is essen- 

 tially the same as that in this chapter. 



Runnstrom's concept of two opposed and overlapping gradients in the 

 apicobasal axis of the sea-urchin egg was originally stated in terms of con- 

 centration gradients of "animal" and "vegetal" substances, but more re- 

 cently it has been maintained that specifically different animal and vegetal 

 metabolisms characterize the axis. The hypothesis of two overlapping 

 gradients was stated as if established fact in the introduction to the first 

 of Runnstrom's experimental studies on sea-urchin development (1914) 

 and has appeared in many papers since."" According to Runnstrom and 

 Lindahl, lithium inhibits the kind of metaboHsm characterizing the ani- 

 mal gradient and stimulates or in some way favors that of the vegetal 

 gradient. As already pointed out, differential susceptibiUty to gradually 

 lethal agents, differential modification of development, and differential 

 dye reduction give no information concerning presence or absence of such 

 overlapping gradients. Moreover, it may be noted that two overlapping 

 concentration gradients of different substances may be associated with 

 a single gradient of metabolic rate; and metabolism, rather than concen- 

 trations of substances, is the effective factor in development of organismic 

 pattern. Under natural conditions concentration gradients do not appear 

 to be the primary factors determining metaboHsm; metabolism is unques- 



•9 Child, 19166; MacArthur, 1924, accepting Child's views. 



2° Runnstrom, 1914, 1915, 1925a, 1928a, b,c, 19296, 1931, 1933, ig^S^, b, etc.; Horstadius, 

 1928&, 1931, 1935, 1936a, b, 1937a, 1938; Lindahl, 1932a, b, 1933, 1935, 1936. 



