THE AXIAL GRADIENTS IN HYDROZOA. 



relations in the later bud stages occur, but since they are similar 

 to the modifications in full grown animals under the same con- 

 ditions, their consideration is postponed to a later section. 



THE GRADIENTS IN ATTACHED QUIESCENT ANIMALS. 



For present purposes "attached animals" are animals which 

 have been left undisturbed in Syracuse dishes for several hours, 

 usually over night, and are firmly attached to the glass of the 

 dish. Animals attached to the surface film or to air bubbles are 

 not included. After the preliminary period of attachment, most 

 of the water is carefully drawn off, the agent added with a pipette 

 and the dish covered without air bubbles, great care being taken 

 to disturb the animals as little as possible. Since cyanide is the 

 only agent among those used in this study which does not pro- 

 duce some considerable degree of excitation, it is employed in 

 various concentrations from m/i,ooo to m/2oo with the most 

 satisfactory results, where it is desired that the animals shall 

 remain quiescent. The change of fluids is often accomplished 

 without inducing any strong contraction, and if the animals con- 

 tract at all, they usually extend again at once or remain at least 

 moderately extended until death, unless otherwise disturbed. 



Under these conditions individuals of H. viridissima and H. 

 vulgaris show the primary gradient, disintegration progressing 

 basipetally throughout tentacles, body and stalk (Figs. 32-35). 

 In cases where the experiment is not entirely successful and any 

 considerable degree of contraction of the stalk occurs, disinte- 

 gration progresses basipetally as before in tentacles and body, but 

 as disintegration of the hypostome begins, or some\vhat later, 

 disintegration also begins in the stalk somewhere above the foot 

 and progresses in both directions (Fig. 36), or in some cases at 

 the foot itself and progresses acropetally (Figs. 37, 38), death 

 occurring last in or just apical to the basal region of the body (Fig. 

 39). Figs. 40-42 show a particularly clear case of this sort in H. 

 vulgaris. Here the attached animal remained extended during 

 the disintegration of tentacles and hypostome and the usual 

 basipetal gradient appeared (Fig. 40), but at a later stage the 

 stalk underwent considerable contraction (Fig. 41), probably in 

 consequence of movements of the dish to and from the microscope 



