116 STUDIES IN GENERAL PHYSIOLOGY 



it is possible so to change the form of the animal that it ter- 

 minates at both ends either in oral (Fig. 17) or aboral poles, 

 and yet continues to live. On the other hand, I have found 

 animals in which all attempts at the transformation of organs 

 have thus far been unsuccessful. To this group belong 

 Cerianthus membranaceus and many other Actinians. I 

 succeeded, however, in bringing about a permanent change 

 of form in one of these animals (Cerianthus membranaceus), 

 in which I was able to cause the growth of any number 

 (within certain limits) of mouths, one above the other, in one 

 and the same animal. 



The regeneration of lost organs in animals has often been 

 made the subject of study, usually, however, only to see 

 which organs can be regenerated, and further to study more 

 closely the anatomical or histological details of the process 

 of regeneration. But it has rarely been considered that 

 these phenomena can give us an insight into the conditions 

 that control the morphogenesis of animals. Where this has 

 been done, it has almost always been with the intention of 

 showing that under all conditions only one and the same 

 organ grows from any definite point on the animal. 



Allman 1 was perhaps the first to define this sharply as a 

 law of the formation of organs. From the well-known 

 experiments of Trembley, 2 Dalyell, 3 and from his own obser- 

 vations, he formulated the theory of the "polarity" of the 

 animal body. Allman cut pieces from the stem of Tubu- 

 larians and marked the end which had been directed toward 

 the head of the animal. Even though this cut end was mor- 

 phologically entirely similar to the other, yet a head was 

 formed only at this oral end, while no head was formed at 



i GEORGE J. ALLMAN, Report of the British Association for the Advancement 



) 1864. 



2 A. TREMBLEY, Memoires pour servir a Vhistoire d'un genre de polypes d'eau 

 douce a bras en forme de cones (Leide, 1744). 



3 J. G. DALYELL, Rare and Remarkable Animals of Scotland (London, 1847). 



