80 JOHN W. MACARTHUR. 



and the feature by which the lithium effect may best be quantita- 

 tively estimated. The greater the lithium effect short of general 

 inhibition, the greater thelrelative size of the entodermal as 

 compared with ectodermal parts. 



Meanwhile mitosis and growth have slowed down or utterly 

 ceased in the ectodermal zone, whose development varies in- 

 versely with the development of the entoderm. In extreme cases 

 the entodermal growth zone overspreads the whole blastula wall 

 resorbing and incorporating and changing entirely the prospec- 

 tive significance of the apical region. This resorption doubtless 

 proceeds in proportion to the relative rate of metabolic activity of 

 the entoderm; just as in normal ontogeny the ectoderm feeds 

 upon and limits the development of the gut, or as the nervous 

 system of a starving animal maintains itself at the expense of 

 parts of lower physiological activity. Here the ectoderm, usu- 

 ally the master germ layer, loses control and the more active 

 entoderm gains the ascendency and may become the major part 

 of the embryo. That the fore-gut of the evaginated archenteron 



also increases disproportionately may be attributed to the same 

 process. 



The inhibitory effect of lithium appears in the order: ecto- 

 derm > entoderm > mesenchyme; in the ectoderm itself: api- 

 cal > basal ;*and in the entoderm: hind-gut > mid-gut > fore- 

 gut (Fig. 13). 



The most substantial direct proof of this reversal of the normal 

 physiological gradient is found in the more or less complete re- 

 versal of the usual order of susceptibility; for in extreme cases 

 the direct susceptibility was seen to be greatest in the fore-gut 

 region and to decline towards the hind-gut and ectoderm (Fig. 13). 



Judged then by the criteria of susceptibility, of developmental 

 activity in mitosis, growth, rate of differentiation, etc., the meta- 

 bolic gradient of the lithium larva is diminished, obliterated, or 

 in the extreme case actually reversed (basi-apical). It is evident, 

 however, that the reversal of the gradient occurs too slowly or 

 too late to alter significantly the nature of the differentiated end 

 products, for the respective germ layers bear recognizable simi- 

 larity to these of the normal larva. Earlier exposure of the egg 

 and a different technique of recognizing polarity would be neces- 



