7. Restitutionslehre. 311 



rate or intensity of the processes in the anterior region. Moreover, not only 

 is the head region dominant over the regions posterior to it and within the 

 limit of effectiveness, but in general a given level of the body is dominant 

 over regions posterior to it within the limit and is dominated by regions 

 anterior to it." 



„A piece above a certain size from any level of the body is capable of 

 reproducing all parts which lie posterior to its level whether it produces a 

 head or not, but it is incapable of giving rise to any of the parts which lie 

 anterior to its level, unless some approach to head formation occurs first." 



This is illustrated by a series of experiments. The dominance of the 

 head region is also shown by the fact that the position of regenerating organs 

 such as the pharynx and intestinal branches in the regenerating part is a 

 function of the relative size of the head, whether the Variation in size of the 

 head be a function of position of the regenerating surface in the original 

 animal, or of external conditions. For instance, if the growth of the regene- 

 rating head be inhibited by depressing agents, such as alcohol, the proportion 

 of the pre-pharyngeal region to the whole regenerating part is diminished, etc. 

 It is thus possible to alter the location of the pharynx within very wide 

 limits. It also follows that „any condition which retards or inhibits the 

 development of the head or the dynamic processes in the developed head 

 favors the development of a second zooid at the posterior end and any con- 

 ditions which accelerate the dynamic processes in the anterior region retard 

 the development of the second zooid." 



The head is also dominant in starvation and becomes relatively larger 

 the farther the reduction proceeds. „I believe that we have here a very 

 general law of development which has not been clearly recognized." 



IL The Eegulatory Development of the Anterior End. 



In pieces below a certain size, which varies with the region of the body 

 and with other internal and external conditions, normal wholes do not regene- 

 rate. These incomplete forms are classified as: (1) Tailless, characteristic of 

 very short pieces from the rnore anterior regions; (2) Teratophthalmic, in- 

 volving abnormalities of the eyes leading to cyclopism; (3) Teratomorphic, 

 usually cyclopean with reduction of the median anterior region; (4) Anophthal- 

 mic, without eyes; (5) Headless. 



III. Experimental Control of the Character of the Anterior End. 



„For given conditions a certain rate of reaction is necessary for the for- 

 mation of a normal head. When the rate falls below this critical level 

 teratophthalmic heads appear first, then as the rate falls still further the 

 teratophthalmic and anophthalmic and headless forms appear in succession." 



The factors studied are size, region, temperature, alcohol, nutrition and 

 mechanical Stimulation. These are shown by selected series of experiments to 

 determine the occurrence of the various classes of abnormalities. Perhaps the 

 most interesting results are those under mechanical Stimulation, in which it 

 was shown that of a large number of comparable pieces divided in two sets 

 one of which is repeatedly stimulated mechanically so as to induce motor 

 activities as much as possible, the other set being left quiescent under other- 

 wise identical conditions, the amount and normality of regeneration is much 

 greater in the former. 



„From the morphological point of view the most important point is that 

 certain organs may alter their localization, their form and their relation to 

 each other, and may finally disappear as the rate of the processes concerned 

 decreases and vice versa. It is evident that under given conditions a certain 



