GRADIENT-FIELDS 47 



were associated with two different metabolic processes, viz. the 

 animal gradient with carbohydrate metabolism, and the vege- 

 tative gradient with protein metabolism. Lithium was found to 

 inhibit carbohydrate metabolism. This would explain its in- 

 fluence on the development of the embryo. However, attempts 

 at a direct demonstration of metabolic differences between the 

 animal and vegetative halves remained unsuccessful. Moreover, 

 recent investigations, mainly those of Horstadius and Gustaf- 

 son (1950), indicate that lithium may also influence the syn- 

 thesis of proteins. Certain cell enzymes may be the primary point 

 of attack, and its influence on metabolism may be secondary. 



On the other hand, the work of Ranzi and his collaborators 

 has shown that substances which in vivo influence the de- 

 termination of the sea urchin's egg, i7i vitro modify the 

 structure of various proteins with long chain-molecules. Lithium 

 and other substances which reinforce the vegetative gradient 

 of sea urchins' eggs were shown to effect condensation of these 

 proteins. Substances such as thiocyanate, which favour the 

 animal trends in development, in vitro caused a loss of fibrillar 

 structure in the proteins. From these facts, Ranzi (1947) con- 

 cluded that these fibrous proteins play an important role in the 

 determination process. 



The egg of the snail Limnaea stagnalis furnishes another 

 example of the influence of gradient-fields in development. 

 Here again, treatment with lithium chloride was found to cause 

 characteristic disturbances in development (Raven, 1948, 1949). 

 It results in embryos in which the middle part of the head is 

 reduced so that eyes and tentacles (normally paired) unite 

 on top of the head (so-called cyclopia). In normal development 

 the parts that are suppressed by lithium develop from the cells 

 lying around the animal pole of the egg. Here, just as in sea 

 urchins, lithium evidently inhibits an animal gradient-field 

 which plays a role in normal development. In accordance with 

 Ranzi's views, lithium proved to have a condensing influence 

 on the cytoplasm of the egg in this case as well. On the other 

 hand, it caused increased swelling of the nuclei. 



If the polarity of the egg is determined by axial gradients, 

 disturbance of these gradient systems may be expected to 



