DYNAMICS OF REGENERATIVE PROCESSES 217 



fore arm, the hand, and the leg from the knee down may suddenly be- 

 gin to grow even if the body had already reached its final size. This 

 phenomenon, known as acromegaly, is comparable to the process of re- 

 generation, inasmuch as the problem in this case is also what causes 

 this sudden growth which normally does not occur. It has been ob- 

 served that this disease is often connected with a degeneration of the 

 hypophysis, a small organ of enigmatic function. An attempt has been 

 made (Von Cyon) to cure this disease by feeding the patient on normal 

 hypophyses. This idea is based on experiences made in regard to the 

 thyroid gland. Complete extirpation or degeneration of the thyroid gland 

 in growing persons causes a standstill of growth and sexual and mental 

 development. It has been found that if such patients be fed on thyroid 

 glands, the growth and development can be again started. This seems 

 to speak in favor of the idea that the action of specific substances may 

 cause the post-adult growth of arms and legs in the case of acromegaly. 

 It may, however, be a case of growth due to nervous influences com- 

 parable to the experiences of Przibram in Alpheus. 



6. THE EFFECT OF SOME EXTERNAL CONDITIONS UPON REGENERATION 

 AND THE TRANSFORMATION OF ORGANS 



If we cut a piece ab, Fig. 59, from the stem 55 of Antennularia an- 

 tennina (Fig. 60), a hydroid, and put it into the water in a horizontal 

 position, new stems cd, 

 Fig. 59, may arise on its 

 upper side. The small 

 branches on the under 

 side of the old stem ab 

 begin suddenly to grow 

 vertically downward.* In 

 appearance and function ^ 

 these downward-growing 

 elements are entirely dif- 

 ferent from the branches 

 of the normal Antennu- 

 laria ; they are roots. FlG 

 In order to understand 



better the transformation which thus occurs in these branches, it may 

 be stated that under normal conditions they have a limited growth 

 (see Fig. 60), are directed upward, and have polyps on their upper side 



* Loeb, Untersuchungen zur physiologische Morphologic der Thiere, II, 1891. 



