244 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tilization; and with lithium, exogastrulae may develop from eggs treated 

 with thiocyanate before fertilization. According to Lindahl, some of 

 these exogastrulae are bipolar forms united by the distal ends of their ex- 

 ternal entoderms." The possibility that the animalized forms may not be 

 direct effects of the thiocyanate or iodide but secondary modifications re- 

 sulting from recovery after return to water, together with the activation 

 associated with fertilization, is not considered by Lindahl. Some degree 

 of ectodermization or animalization may apparently occur following ex- 

 posure to lithium after fertilization (pp. 227, 235), and the possibility that 

 Lindahl's animalized forms are cases of recovery cannot, at present, be 

 excluded. The fact that cyanide and lithium after fertilization prevent the 

 animalization of eggs treated with thiocyanate before fertilization sup- 

 ports, rather than conflicts with, the view that the animalization repre- 

 sents a recovery, not a direct effect of thiocyanate or iodide. Lindahl 

 finds, also, that temporary exposure to thiocyanate or iodide after fertiliza- 

 tion animahzes in earlier, vegetalizes in later, stages. This is what might 

 be expected with differential recovery following inhibition. 



Following Lindahl's procedure, Rulon (1938, 1940) has obtained ani- 

 malized forms of Dendraster, though less extreme than some of those de- 

 scribed by Lindahl for other species, and suggests that they represent 

 recovery from a primary inhibition. According to this suggestion, thio- 

 cyanate inhibits differentially the slight apicobasal gradient present in the 

 unfertilized egg, so that any dominance of higher gradient-levels or any 

 definite relation between parts is almost or quite abolished; that is, in 

 extreme cases axiate pattern is virtuall}' obliterated. After return to water 

 and fertilization, with the accompanying activation, the regions which 

 would have been the lower gradient-levels and would, therefore, have de- 

 veloped as entoderm are more or less physiologically isolated, since they 

 are in the same, or almost the same, physiological condition as more apical 

 levels. Under these conditions they develop as higher gradient-levels. 

 This may be regarded as essentially a reconstitution similar in principle 

 to reconstitution of an apical region by a physically isolated basal half of 

 a sea-urchin embryo and to reconstitution of apical regions by physically 

 isolated pieces of hydroids and planarians. 



" In Lindahl's figures the supposedly bipolar exogastrulae are identical with cases of union 

 of two exogastrulae by the tips of their entoderms. These appear in large numbers with the 

 more extreme lithium inhibitions, and, as described above (p. 239), many individuals may 

 stick together in this way with gradual obliteration of their individuality. At present it seems 

 not impossible that the "bipolar" forms figured by Lindahl are not actually bipolar, in the sense 

 that they have developed from a single egg, but are two exogastrulae united by the tips of 

 their entoderms. In these unions the blastocoels become continuous by dissociation of the 

 cells in contact, exactly as described by Lindahl. 



