COELENTERATES AND PLANARIANS 215 



complete organism. In the latter case there must be a local factor active, which 

 causes the various organs to reproduce each its own kind, although even under 

 these conditions inducting factors acting in the direction of the long axis may 

 play a certain part. Likewise the fact observed by Child, that even in the 

 absence of a head an isolated piece of Planaria is able to regenerate all parts 

 representing the levels posterior to its situation in the organism, points to the 

 existence of a predifferentiation in these parts, and there are some indications 

 that it is the nervous system which plays, here, an important role in deter- 

 mining the rudimentary differentiation. 



In Planaria, as in a similar manner in hydrozoa, it is possible to demon- 

 strate the existence of organizers. In both classes of organisms it is especially 

 the most differentiated organ area, the anterior pole or head, which not only 

 dominates the structure of the organism, but may also act as organizer (Child, 

 Goetsch, Santos). Furthermore, both classes show the same types of induc- 

 tion, and the inducting organ gives origin to its own kind of organ in the 

 material acted upon; in hydrozoa a transplanted head gives rise to a new 

 head, and in Planaria, according to Gebhardt, the eyes of the host may induce 

 eye formation in a bud from the posterior part, which has been transplanted 

 into the head region. In addition, a second type of induction has been estab- 

 lished in Planaria, especially by Morettj, Goetsch and Santos. Goetsch ob- 

 served that a transplanted head can induce in the host a reorganization, which 

 leads to the development of a postcephalic region. Santos grafted a piece from 

 the ganglion region of Planaria into the prepharyngeal levels of the host. If 

 the transplant was of a sufficient size, it gave rise to a head and determined 

 in the host a postcephalic outgrowth. If implanted into postpharyngeal levels 

 of the host, the transplant not only determined postcephalic outgrowth in the 

 host, but, besides, it caused a further reorganization, with the development 

 of a pharynx and postpharyngeal region. But the reorganizing influence of 

 the grafted piece extended in the host also in an anterior direction and in 

 this way it could determine a reversal of the polarity. However, the host. too 

 may exert an influence on the grafted part. This was indicated by the fact 

 that when the union between host and transplant was complete, the host 

 inhibited the perfect development of a head from the graft, while an incom- 

 plete union gave the transplant a chance to develop in accordance with its 

 own potentialities. 



As Rand has found, the inhibiting influence which a graft exerts on a 

 wound, in a more or less specialized region in the host, varies somehow in 

 an inverse relation to the distance of the inhibiting material from the wound 

 surface. This suggests that we may have to deal with a diffusible contact 

 substance, which decreases in concentration with the distance between graft 

 and wound. Besides such inhibiting effects, we have then to deal here, as in 

 coelenterates, with two kinds of actions. One leads to the reproduction of the 

 same organ as that which acts as an organizer and the second represents a 

 complementary, integrative mechanism, which causes the completion of a 

 whole organism from a part. 



We may assume that the inducting action of the transplanted head region 



