DYNAMICS OF REGENERATIVE PROCESSES 



205 



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 b, Fig. 45). In this space a 



circulation of the liquid is maintained by the 



ciliary motion of the endothelial cells in such a 



way as to make the flow of liquid 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, 

 e.g., 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 established. This may be due to the fact that the 

 circulation serves as a means of supplying oxygen, and a 

 liberal 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- 

 i ^ culation can form a polyp. Thus, when a stem is split 

 U lengthwise, the cellular masses may become disconnected 

 FIG. 45. into several pieces. Each such piece may form a polyp at 



both ends. 



When we ligature a stem in the middle, the circulation will be inter- 

 rupted, inasmuch as at a place where the ligature is made the septum 



* E. Bickforcl, loc. cit. 



FIG. 44. AFTER Miss 

 BICKFORD. 



The same specimen a 

 little later, after the 

 formation of the 

 polyps was com- 

 pleted. The chitinous 

 tube was removed. 



