. The dynamics of living matter . of a polyp inside of the intact stem, but only where the cells inside the chitinous layer come in contact with the sea water. The mere interruption of the continuity of a stem does not seem to suffice, as I have never seen the formation of a polyp at the point where the stem was ligatured. It is possible that the lack of oxygen is responsible for the fact that no polyp can be formed except at a free end of a stem, since the chitinous surface of the stem is very little permeable for oxygen. A second condition is the closing of the body cavity after the cut is m
. The dynamics of living matter . of a polyp inside of the intact stem, but only where the cells inside the chitinous layer come in contact with the sea water. The mere interruption of the continuity of a stem does not seem to suffice, as I have never seen the formation of a polyp at the point where the stem was ligatured. It is possible that the lack of oxygen is responsible for the fact that no polyp can be formed except at a free end of a stem, since the chitinous surface of the stem is very little permeable for oxygen. A second condition is the closing of the body cavity after the cut is made and the establishment of a circulation. The body of the Tubularian hydroid is a long, hollow cylinder, and the hollow space in the center of a Tubularian is divided by a septum into two chambers which communicate at the ends {a and h, Fig. 45). In this space a circulation of the liquid is maintained by the ciliary motion of the endothehal cells in such a way as to make the flow of Hquid ascending on one side and descending on the opposite side. This stream carries red and yellowish pigment granules which are apparently formed by the cells of the en- dothelium. The analogy with similar cases suggests the possibility that these pigments are respiratory pigments, serving a purpose similar to, , the red blood corpuscles in our body. If we now cut out a piece from the stem of the hydroid, the opening at either end closes in an hour (more or less according to the temperature) by the cells of the cut end spreading out over the opening concentrically.* According to my observations, no piece of a hydroid can regenerate unless this circula- tion is estabhshed. This may be due to the fact that the circulation serves as a means of supplying oxygen, and a Hberal supply of oxygen seems to be an absolute prereq- uisite for all phenomena of regeneration and growth. Any piece of a Tubularian, however, which can establish a cir- culation can form a polyp. Thus, when a stem is spht length
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