Pressure upon the Regeneration of Gunda ulvse. 



7 



regulation is never larger than the fragment which produced it. Fig. 5, 

 1-4, show the stages in the transformation of an anterior end into a complete 

 worm. It can be seen that there is in this case actual diminution in length. 

 This is due to the fact that the regulation occurred under starving conditions. 

 When the pieces are fully fed there is no reduction in length. 



i 



Fig. 5. — Kegeneration of Posterior End in Sea- water. 

 1. 7 days. 2. 14 days. 3. 28 days. 4. 49 days. 5. Side view of 3. 



The chief role in the transformation is played by the parenchyma cells. 

 These cells are small migratory cells which are found in large numbers 

 throughout the whole length of the animal's body. In regulation they have 

 two functions — 



(1) They migrate in large numbers to the region of the wound, where they 

 form first an undifferentiated mass of cells and later the new tissues. 



(2) They act as phagocytes, making a sheath round the old organs, e.g., 

 brain and genitalia, and reducing them in size until they have become 

 proportionate to the size of the new worm. 



The course of regulation in O. ulvcc is essentially similar to that already 

 described in various species of Planaria by Schultz (11), Flexner (4), and 

 Bardeen (1). The wound is closed by the ectoderm creeping inwards as a 

 thin acellular layer (see fig. 6, ac.ep). 



After the wound has healed (three to five days after section) the 

 parenchyma accumulates at the hind end. This accumulation of parenchyma 

 cells is chiefly due to migration. Mitotic figures are to be found, but are 

 not common in the sections, showing that actual cell division plays only a 

 small role in the building up of the new tail. This consists for a few days of 

 undifferentiated parenchyma, but later differentiated cells appear. The 

 formation of the muscular layer of the new tail takes place by differentiation 



