FIELDS AND GRADIENTS IN NORMAL ONTOGENY 337 



inhibition causing a flattening of the primary (animal-vegetative) 

 gradient of the egg, the apical portions being more susceptible to 

 lithium. However, it cannot be due entirely to this, since differ- 

 ential inhibition brought about by KCN does not result in a rela- 

 tive increase of endoderm. There must be some more specific effect 

 of the lithium, though this again is not purely specific, since similar 

 exogastrulae can be obtained by treatment with the salts of other 

 alkali metals such as potassium, and such substances as carbon 

 monoxide. Examination of sea-urchin eggs under dark ground 

 illumination has revealed the presence of a yellow-coloured ring, 

 the extent of which appears to coincide with the presumptive endo- 

 derm. Treatment with lithium raises the upper border of this ring 

 towards the animal pole and thus provides a visible index of the 

 degree of " endodermisation". The effect of lithium appears to be 

 exerted on the colloid structure of the cytoplasm, which it coarsens; 

 and since in normal development the ectoderm cells present a 

 finer microstructure than the endoderm cells, it is probable that 

 this coarsening renders differentiation along ectodermal lines 

 impossible.^ (See also Appendix, p. 496.) 



A remarkable contrast to the '' vegetativised " larvae produced by 

 lithium are the "animalised" larvae which result from a treatment 

 of the unfertilised eggs with sodium thiocyanide (NaSCN). Such 

 larvae show an expansion of the ectodermal region at the expense of 

 the endodermal : the cilia of the apical organ occupy more than the 

 normal area ; the gut is smaller or even absent ; and the number of 

 skeletogenous mesenchyme cells is reduced, even altogether to 

 zero.^ 



In such larvae which are completely "ectodermalised", a very 

 interesting feature is the appearance of a second apical organ at the 

 vegetative pole: in other words, the original animal-vegetative 

 gradient has been steepened, and the secondary vegetative-animal 

 gradient obliterated: its place has been taken by an additional 

 gradient of the animal-vegetative type, but with its apical point on 

 the site of the vegetative pole. The polarity of the vegetative half of 

 the egg has been reversed, and the larva is comparable to a biaxial 

 head-regeneration in Planaria (p. 285). 



If now such an "animalised" larva is subjected to lithium treat- 

 ^ Runnstrom, 1928. 2 Lindahl, 1933 c. 



