184 C. M. CHILD AND L. H. HYMAN. 



Hydra, H. viridissima (viridis}, H. vidgaris (grisea) and H. oli- 

 gactis (fusca) and data on several hundred individuals have been 

 recorded. The stages used include full grown asexual animals 

 with and without buds, and all stages in the development of 

 buds, and a few observations have been made on sexual individ- 

 uals, animals developed from the regulation of pieces, and ani- 

 mals reduced in size by starvation. 



The observations on H. oligactis, begun in 1913 by Child, have 

 been repeated several times since on different stocks of animals 

 and have been confirmed during the present year by Hyman. 

 The observations on H. viridissima and H. vulgaris were first 

 made by Hyman during 1918 and have been confirmed by Child. 

 All important points concerning the gradients in the three spe- 

 cies rest on observations made independently by both of us. 



. For the work on susceptibility, Syracuse dishes with covers of 

 very thin glass are used. The animals are placed in the dish in 

 a little water, or in certain experiments most of the water pre- 

 viously present is removed with as little disturbance as possible, 

 the agent, freshly made up to the desired concentration, is added 

 at once, or allowed to flow gently in from a pipette until the dish 

 is quite full, and the dish is then covered with exclusion of all 

 air bubbles. This procedure prevents loss by volatilization and 

 makes it possible to examine the animals at all stages under a 

 low power of the compound microscope, and the dish can be 

 moved if some care is exercised, without disturbing the animals 

 sufficiently to produce contraction. 



STRUCTURAL AND FUNCTIONAL SPECIALIZATION OF BODY REGIONS 



IN THE THREE SPECIES. 



The four parts, tentacles, hypostome, column and foot present 

 of course the same general characteristics in all three species, but 

 there are .certain features of the different body regions and cer- 

 tain minor differences between the species which are significant 

 in relation to susceptibility. 



In the tentacles of all three species the capacity for elongation 

 and contraction apparently decreases basipetally, but the ten- 

 tacles of H. oligactis in the stocks with which we have worked, 

 are much longer and capable of a much greater degree of exten- 



