RECONSTITUTIONAL PATTERNS IN EXPERIMENT 431 



nutritive agar, or even a purely mineral substrate. In general, the less 

 illuminated side or the side toward the earth becomes ventral, but the 

 side toward the substrate becomes dorsal. In Marchantia, however, the 

 effect of light depends on temperature. With germination at high tem- 

 perature the less illuminated side tends to become dorsal instead of ven- 

 tral ; also, with light and gravity acting in opposite directions, the effect 

 of gravity tends to overbalance that of light at high temperature. With 

 equal action of external factors in opposite directions on the two sides 

 both may differentiate in the same way (Fitting, 1935, 1937). In some 

 of the higher plants dorsiventrality may be determined by Hght, and 

 gravity may also be a factor. It is an interesting question how the region 

 of the circumference of the plant vegetative tip where the first lateral bud 

 primordium develops, or the first two, three, or more in forms with oppo- 

 site or whorled lateral buds, are determined. Perhaps a more complete 

 acquaintance with botanical literature would show that this question has 

 been answered, but essentially similar questions regarding animal develop- 

 ment await an answer. 



Physiological, rather than external, factors are usually concerned in 

 localizing agamic reproductions of new individuals and development of 

 parts and organs in animals, but there are some cases in which external 

 factors may be concerned and regarding which the question raised above, 

 concerning lateral bud primordia in plants, must be asked. For example, 

 how is the region of the circumference determined where the first lateral 

 bud appears on a Hydra individual? The same question arises with re- 

 spect to the first medusa bud on the manubrium of Pennaria or other 

 hydroids. In development of the hydranth from the planula of Corymor- 

 pha a single tentacle often appears before others. How is its localization 

 on the circumference of the planula determined? This planula does not 

 swim but creeps on the substrate with one side in contact; the first ten- 

 tacle often develops on the upper side, but that it always does is not cer- 

 tain. In other individuals two tentacles on opposite sides, or three, equi- 

 distant from each other, develop apparently simultaneously. The apical 

 region of the planula often turns away from the substrate before tentacles 

 develop; but even if equal exposure determines simultaneous develop- 

 ment of two or three tentacles, the question of how they are localized on 

 the circumference remains. With localization of one Hydra bud, medusa 

 bud, or tentacle bud, its local dominance may play a part in determining 

 localization of another by determining that another cannot develop within 

 a certain distance from itself. A question of somewhat different character 



