MORPHOLOGY AND PHYSIOLOGY OF GENUS URONYCHLi 371 



Three of these fragments were from the anterior end of the ani- 

 mal, and it was easily seen that the posterior cirri which they 

 formed were not normal. In at least one the full number of 

 cirri did not form, and the arrangement was therefore unusual. 

 The one posterior amicronucleate piece which did not complete 

 regeneration died at the end of about twenty hours. This may 

 have been the result of mechanical injury. Eighteen experi- 

 ments showed complete regeneration in both fragments, although 

 the ones without micronuclei invariably died without dividing. 

 These pieces lived sometimes five days. The parts with mi- 

 cronuclei completed regeneration and were, to all appearances, 

 normal at the beginning of division. However, in four instances 

 the division resulted in the formation of cells of unequal size. 

 Summary of table 6. Nineteen U. setigera were cut into very 

 unequal parts from one hour after division up to the time of the 

 following division. In four of these experiments both parts 

 died, but not until after regeneration had begun in all. The 

 plane of cutting in each instance was such that it probably in- 

 jured or destroyed the micronucleus. In four other cases the 

 smaller parts died very soon after the cutting, evidently be- 

 cause of the mechanical injury. In all of these instances the 

 larger part regenerated completely and divided. In eight other 

 experiments the smaller piece did not regenerate. The larger 

 part invariably did regenerate and divide. In only two cases 

 did the smaller parts regenerate completely, but these parts did 

 not divide. In both of these the individuals formed from the 

 small pieces were minute (in length only 10 to 15 /x), but nor- 

 mal in every respect as far as could be seen. In three of the 

 cases mentioned above, the division of the part with micronu- 

 cleus resulted in the formation of two individuals of unequal 

 size, just as was observed in those instances listed in table 5. 

 This unequal division indicates that a 'division zone' is formed 

 during the growth of the cell long before the actual process of 

 division begins. 



