430 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tical area with gradient of potency for gastrulation and inductor develop- 

 ment decreasing from the mid-dorsal region of the gray crescent in the 

 frog egg, that is, from the region of primary invagination in normal de- 

 velopment. They regard the metabolisms resulting from, and determined 

 by, local concentrations of cytoplasm and yolk and from relative differ- 

 ences in amounts of each as essential factors in development. In the 

 single, double, or triple embryos and monsters developing from these in- 

 verted eggs polarity, symmetry — in fact, the whole axiate pattern — may 

 be the result of a reconstitution and entirely a new pattern ; but the locali- 

 zation of new dorsal lips, blastopores, and neural plates shows a relation 

 to the original dorsal region, suggesting a graded differential in that re- 

 gion. It is of some interest to note that results of experiments on differ- 

 ential susceptibihty,. differential dye reduction, and distribution of SH- 

 proteins and some of the data on oxygen consumption are in general 

 agreement with these conclusions (see. pp. 151-58). 



As regards the vegetative reproductions of parts in plants, it was 

 pointed out above that external factors may determine the regions in 

 which certain parts develop, but not their axiate patterns. It has long 

 been known that pattern of branching in the thalli of certain algae can 

 be determined by direction of illumination in relation to the main axis 

 of the thallus. Branching is radially symmetrical when illumination is 

 equal on all sides, bilateral or dorsiventral when it is unequal (Berthold, 

 1882). Other external factors — for example, gravity — may affect pattern 

 of arrangement of axes in many plants though not determining the pat- 

 tern of the particular axiate parts. In various plants, however, external 

 factors may determine dorsiventrality directly. For example, it is well 

 known that light can determine dorsiventrality in certain algae, mosses 

 and liverworts, and in the prothallia of ferns. In some of these forms a 

 dorsiventrality once determined can be reversed in the later growth by 

 reversing direction of illumination; in others it is stable. Rhizoids and, 

 in fern prothallia, sex organs develop from the side regarded as ventral, 

 and in many liverworts the thallus itself becomes dorsiventrally differ- 

 entiated. It has been shown for many of these forms that the entire dorsi- 

 ventral pattern of further growth is experimentally reversible by reversal 

 of illumination. The gemmae of various liverworts possess bipolar pat- 

 tern with apical growing cell at each tip and each gives rise to two thalli 

 that grow in opposite directions and finally become separated. Extensive 

 experiments with gemmae of Marchantia and Lunularia have shown that 

 their dorsiventrality can be determined by light, by gravity, or by a 



