324 University of California Publications. [ ZcoLoGy 
two to four times as long as broad. They ultimately grade into 
the internodal segments of the stem, each giving rise at its distal 
end to one hydrotheeal pedicel. The pedicels themselves vary in 
length and annulation from the base of the stem toward the tip. 
The first pedicel may exhibit 4 to 7 annul. The last hydro- 
theca may possess no pedicel at all, or one of but a single an- 
nulus. The number to a pedicel varies with much general uni- 
formity from one of these extremes to the other. 
From base to tip, then, each stem presents two main serial 
changes. (1) Below the hydranth region, the cauline annuli 
erow progressively longer. (2) Within the hydranth region, the 
number of annuli to a pedicel progressively decreases. There 
is a strong suggestion in these facts that they depend on changes 
in the internal physiological conditions of growth. Since polyps 
are readily regenerated where stems are sectioned, a method at 
once presents itself for determining the differentiation at a given 
level of the stem at a given time. 
The question to answer which the investigation was first un- 
dertaken was: Will the stem at a given level tend to regenerate 
the structural type which it originally produced there; or will 
it produce instead, a structural type characteristic of another 
region? For example, will a cut through the portion of the stem 
where each hydrothecal pedicel has five annuli initiate the re- 
generation of a pedicel with five or with one annulus, the number 
characteristic of the distal region ? 
The facts obtained from the experiments fall into three cate- 
gories, according as the regeneration followed (1) a cut through 
the distal portion of the stem, (2) a cut through the middle por- 
tion, (3) heteromorphically from the cut basal end. 
In all, 16 operations were performed. Of these, 7 were unsue- 
cessful. Among the remaining 9, the cut in 5 had been made 
through the distal region, 3 through the middle region, and 6 
had regenerated heteromorphic stalks. The small number of 
cases involved is offset by the clearness of the results. 
The record of experiments in the first category is as follows: 
I—July 26, No. 2. Stem with 24 hydranths and terminal 
bud. Enumerating from base to tip, 
