THE AXIAL GRADIENTS IN HYDROZOA. 1 95 



gration of the stalk may begin at the base and progress acropet- 

 ally, or may occur at once throughout most of the length of the 

 stalk, but it is evident in all such cases that the stalk has become a 

 region of relatively high susceptibility. In Figs. 20-22 a case of 

 this sort in H. vulgaris is shown. In the earlier stages of disinte- 

 gration (in KNC) the bud remained extended (Fig. 20) but later 

 underwent considerable contraction, the stalk region contract- 

 ing more than the body, and soon after the contraction the stalk 

 began to disintegrate (Fig. 21), and in later stages disintegration 

 progressed basipetally in the body and acropetally but more 

 slowly from the stalk (Fig. 22), so that the two gradients met near 

 the base of the body. Figures 23 and 24 show a similar case in 

 H. viridissima. Here the contraction occurred shortly after the 

 parent and bud were placed in the KNC, but, as in the preceding 

 case, the stalk appears as a region of high susceptibility, and two 

 gradients of disintegration progressing in opposite directions re- 

 sult. These cases are merely examples of what has been observed 

 by us repeatedly in all three species. 



In Figs. 25-27 an animal (H. oligactis] with two buds, one less 

 the other more advanced, is shown. Here the more advanced 

 bud, which has already developed independent motor activity, 

 shows contraction of the stalk and double gradients, as in the 

 cases of Figs. 20-22 and 23, 24. The earlier bud, however, which 

 is much less motile and less independent of the parent body, 

 shows an acropetal instead of the usual basipetal disintegration 

 gradient. This is one of the exceptional cases referred to above. 

 It will be noted that the stalk of the parent is strongly contracted 

 and shows a high susceptibility. The reversal of the gradient 

 in the earlier bud results from the spreading or irradiation into 

 the bud of the excitation connected with the contraction of the 

 stalk. The more advanced bud is more independent, i. e., its 

 own gradient is more fixedly established and it is less intimately 

 connected with the parent, so that is it less affected by contrac- 

 tion of the parent stalk, although the contraction and early dis- 

 integration of its own stalk may perhaps be the result of irradia- 

 tion of the excitation from the parent. Figs. 28-31 show a sim- 

 ilar case of reversal of a bud gradient in H. vulgaris. 



Reversals of this sort in buds have been repeatedly observed 



