/O REGENERA TION 



produce new heads two or three times and then cease, and if after the 

 last-formed heads have died, pieces of the stem are cut off, they close 

 as readily as do pieces from fresh hydroids. Moreover, at certain 

 times of year the species Tubularia (Paryfi/ia} crocea lose their 

 heads, and only the stalks remain. Pieces of these stalks will not 

 regenerate new heads, at this time, although they close in as quickly 

 as do pieces at other times of the year when the heads are present 

 and when new ones regenerate. 



Another equally good illustration of what seems to be the same 

 phenomenon is found in the closing in of wounded surfaces in the 

 young tadpole embryos. If embryos are taken from the jelly mem- 

 branes, or even after they have been set free, and cut in half, each 

 piece quickly covers over the wounded surface by means of the ecto- 

 dermal cells. A much more striking illustration of this closing over 

 in the young tadpole is obtained by cutting, with a pair of small scis- 

 sors, a large piece from the side. The area may be a fourth or more 

 of the entire side, and yet it may be closed over in an amazingly short 

 time. Half an. hour or an hour often suffices to cover a large exposed 

 surface. In this case also the wound is covered not by individual 

 cells wandering over the exposed surface, but by a steady advance of 

 the smooth edge of the ectoderm toward a central point. The process 

 is so similar to that which takes place in tubularia that little doubt 

 can remain as to the two being due to the same factors. As there are 

 no muscle fibres present in the part of the frog's embryo from which 

 the piece is cut off, the result cannot be due to muscular contraction, 

 but appears to be a contractile phenomenon similar to that in tubularia. 

 Even the small piece that is cut from the side of the body shows the 

 same phenomenon. At first it suddenly bends outwards owing to some 

 physical difference between the inner and the outer parts of the 

 piece. Then the edges thicken, bend in, and begin their advance over 

 the inner tissues. The process is seldom completed, since there 

 appears to be a limit to which the ectoderm can be stretched as the 

 edges advance. A most striking phenomenon both in pieces of tubu- 

 laria and of the frog's embryo is the entire absence of dead material 

 at the wounded surface. No sooner is the operation performed than 

 the advance begins,- and there is not a trace of dying cells or parts of 

 cells to be seen. 



