CONTROL OF MORPHOGENESIS IN PLANARIA. 329 



For reasons which I shall state later it seems probable that the 

 factor of length of the piece is essentially quantitative in its 

 effect upon head formation, and the regional factor is certainly 

 very largely quantitative. The effect of alcohol and other anaes- 

 thetics is also quantitative, at least in large part. 



The results obtained in these experiments point to the possi- 

 bility of controlling and analyzing morphogenesis in a great variety 

 of ways and of obtaining some insight into the nature of morpho- 

 genic processes themselves. As regards the data presented, 

 there are many facts which indicate that the rate of oxidations 

 or of certain oxidation processes is the chief factor in the results 

 obtained. 



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 rate of certain 

 dynamic processes is necessary for the production of certain 

 morphological effects. 



IV. THE EFFECT OF CHANGE OF POSITION OF PARTS 



IN THE BODY. 



If we cut small pieces from the posterior region of the first 

 zooid such for example as ef, Fig. I, they will give 100 per cent, or 

 nearly of headless forms, If, however, we cut out a large piece, 

 eh (Fig. i) and allow a head to form at its anterior end and then 

 after a week or ten days cut out small pieces just behind the head, 

 we shall find that they now produce 100 per cent, or nearly of 

 forms like Fig. 2, i. e., normal wholes. These pieces represent 

 approximately the region which when taken from the original 

 animal gave rise to 100 per cent, or nearly of headless forms. 

 Their capacity for forming heads has been widely altered by their 

 changed position in the body and more specifically I believe 

 by altered correlation with other parts. The cells of such a 

 region are not directly concerned in the formation of a head, but 

 the more anterior their position in the new individual, the greater 

 their head forming capacity becomes. 



In a similar manner we can decrease the head-forming capacity 



