INDIVIDUATION AND REPRODUCTION 



213 



Consequently the two ends react with equal rapidity, and be^in 

 development at the same time, and neither becomes dominant 

 over the other.' 



Short pieces of this character have never been known to 

 undergo transformation into stolons or stems without hydranths. 

 A stolon or a stem develops only in connection with a hydranth, or 

 with a piece of stem or stolon, and as an outgrowth from it. In 

 other hydroids and in coelenterates in general, as far as they have 

 been examined, the same relations obtain. The apical region can 

 arise independently of other parts, but stems and stolons arise 

 only in connection with other parts and more specifically with 

 parts which represent physiological regions nearer the apical end, 

 rather than with those to which they give rise. 



In the flatworms we find similar relations of parts. Short 

 pieces from the body of Planaria, for example, may develop into 

 single or biaxial heads without any other part of the body. The 

 head of Planaria when separated from the body by a cut at the 

 level a in Fig. 86 may develop a head on its cut surface, as in Fig. 

 87; and short pieces from other regions, such as the piece be in 

 Fig. 86, may give rise to single heads like Figs. 88 and 89, or some- 

 times to biaxial heads with a short anterior body region between 

 them, like Fig. 90. Evidently development of a head from a piece 

 is possible, even in the complete absence of other parts (Child, '11b). 



In Planaria, as in Tubularia, posterior regions do not arise 

 independently of other parts, but always in connection with regions 

 which are more anterior. Any piece of the planarian body is ca- 

 pable of giving rise to all parts posterior to its own level, whether a 

 head is present or not (Fig. 91), but no piece is capable of producing 

 any part characteristic of more anterior levels than itself, unless a 

 head begins to form first. This point is illustrated by F'igs. 91 

 and 92. These pieces represent the region i J in Fig. 86. When such 

 pieces remain headless, as in Fig. 91 , no changes occur at the anterior 

 end except the slight growth of new tissue, the piece does not give 

 rise to a new pharynx, nor does the more anterior region undergo 

 transformation into a prepharyngeal region. At the posterior end. 

 however, a large outgrowth occurs which slowly attains the 



' See Child, '07a, b, c, 'iia, pp. 101-19. 



