THE ORIGIN OF NEW CELLS AND TISSUES 191 



I shall describe the principal changes that have been found to take 

 place. When the anterior end of lumbriculus or of tubifex is cut off, 

 the cut-surface very quickly closes, as a result of the contraction of 

 the body wall. According to some investigators, the circular muscles 

 are chiefly concerned in the closing, but according to others the lon- 

 gitudinal muscles bring about the result. The cut-end of the diges- 



FlG. 57. After Hasse. Regeneration of head of Tubifex rivulorvm. A. Sagittal section of an- 

 terior end. Six days after cutting in two. B. Eleven days after cutting in two. C. Cross- 

 section through new part. Five days after operation. D. Fourteen days after operation. 

 E. Sixteen days after operation. 



tive tract is pulled a little inward, and its end also closes (Fig. 57, A). 

 For a day or two no important changes can be observed to take 

 place, but new ectoderm soon appears over the cut-surface. This ecto- 

 derm arises in all cases from the old ectoderm, and as it increases in 

 amount the old ectoderm is pushed back from over the cut-end, leav- 

 ing a layer composed of a single row of cells over the end. Since 

 nuclei in process of division are rarely present before these initial 



