INDIVIDUATION AND REPRODUCTION 



215 



characteristic structure of a posterior end. Under certain conditions 

 short pieces give rise to biaxial posterior ends, as in Fig. 93. Morgan 

 ('04) has also described biaxial posterior ends from Planaria sim- 

 plicissima. But when such pieces give rise to a head, even though 

 it is of the rudimentary, anophthalmic type of Fig. 92, a new pharj^nx 

 and mouth arise and the anterior region becomes structurally and 

 functionally a prepharyngeal region, as the change in the intestinal 

 branches in Fig. 92 indicates. In some way all the changes in the 

 piece which concern the development of parts anterior to its own 

 level are dependent upon the presence of a head, or, more correctly, 

 of a head-forming region. 



It has also been shown (Child, '13a, '14b, '14c) that the develop- 

 ment of a head on a piece of the planarian body is not the replace- 

 ment of a missing part under the influence of other parts of the 

 piece, but that head formation takes place, if it takes place at all, 

 in spite of the remainder of the piece. The more vigorous the 

 other regions of the piece, i.e., the higher their rate of metabolism, 

 the less likely is the piece to give rise to a new head, and vice versa. 

 On the other hand, the higher the rate in a piece, the more likely it 

 is to produce a posterior end. In short, the development of a new 

 individual from such pieces of Planaria is essentially the same pro- 

 cess as the development of an individual from the egg. It begins 

 with the formation of a head, and the head-region in some way 

 determines the reconstitution of certain parts of the piece into 

 more anterior parts, while other parts persist with more or less 

 change in size and proportion as corresponding parts of the new 

 animal. In the absence of a head-region any level of the body 

 controls and determines the development of all more posterior 

 levels. Much evidence, largely as yet unpubUshed, indicates that 

 similar relations exist in other forms where the development of whole 

 animals from headless pieces occurs. 



These facts force us to the conclusion that in such experimental 

 reproductions there is a relation of dominance and subordination 

 of parts. The apical or head-region develops independently of 

 other parts but controls or dominates their development, and in 

 general any level of the body dominates more posterior or basal 

 levels and is dominated by more anterior or apical levels. 



