78 JOHN \V. MAC-ARTHUR. 



the late blastula, gastrula and larva the susceptibility demonstra- 

 bly declines sharply from ectodermal to entodermal and mesen- 

 chymal regions (p. 74). This is chiefly significant as indicating a 

 corresponding gradient of metabolic activity as a prerequisite 

 to normal development which is further evidenced and mani- 

 fested by structural differences along the apico-basal axis 

 (differences in cell size, cilia, pigment, vacuolization of proto- 

 plasm, etc.) and by functional differences in rate of develop- 

 mental processes (growth, cell division, morphogenesis, and 

 differentiation). 



It is a familiar fact that teratogenic agents and conditions 

 applied to whole eggs and embryos affect different regions of the 

 developing organism differently and to a different degree. On 

 the quantitative metabolic gradient conception regions of great- 

 est susceptibility are subject to the greatest retardation in the 

 developmental processes, while those of lowest susceptibility are 

 correspondingly least inhibited, as is experimentally proven for 

 Arbacia (Child, 'i6a), Annelids (Child, '17), the frog (Bellamy, 

 '19), etc. Thus most teratological forms may be reproduced 

 according to expectation and explained consistently. 



Lithium effects also seem to be produced by a differential in- 

 hibition along a quantitative metabolic gradient. The facts 

 relevant to this interpretation have been collected in the next 

 section. 



2. Analysis of the Lithium Effects. 



Exogastrulation aside, lithium larva? are characterized mor- 

 phologically by the abnormal proportions of their parts, which 

 involve all the germ layers in a manner we may summarize as 

 follows: 



(a} Increase in amount and change in position of the mesenchymc 

 and its derivatives: 



Excess mesenchyme of stereoblastulae (p. 63). 



Excess pigment cells (p. 63). 



Excess skeletal structures, e.g., spicules, arms, radii of arms, etc. 



(Herbst). 

 Conspicuous production of serous fluids and swelling of serous 



cavities in main' annelid, fish, and amphibian embryos. 



