REGULATION AND FISSION IN PLANARIA. 117 



I do not. believe that any other explanation of these remarkable 

 facts can be found. 



The two exceptional cases noted above (Figs. 6, 7, a, />, c] 

 were from worms captured through the ice in January when food 

 was scarce and activity slight on account of the low temperature 

 and hence in all probability no preparation for fission existed. 

 These cases only serve, therefore, to render more probable the 

 interpretation given above. 



In P. siuiplicissiina, which is not known to undergo fission the 

 relative position of the pharynx in pieces as from different levels 

 is, according to Morgan, similar to that in the winter specimens 

 of P. macitlata. 



II. THE SIZE OF THE HEAD AT DIFFERENT LEVELS. 



In 1901 I first noticed that the new head formed was larger 

 in pieces from the anterior part of the prepharyngeal region and 

 from the posterior half or two thirds of the postpharyngeal region 

 than in pieces from the middle region of the body. Moreover, 

 in the pieces from the middle region a considerable amount of 

 new tissue is formed posterior to the eyes while in the anterior 

 and posterior regions the eyes lie almost in the plane of the cut 

 surface. 



If we cut the whole body into equal pieces and compare them 

 we find the size of the head decreasing and the distance between 

 the eyes and the cut surface increasing as we proceed posteriorly 

 toward the old pharyngeal region. Pieces just anterior or pos- 

 terior to or in the pharyngeal region are much alike, all forming 

 relatively small heads and relatively long " necks " of new tissue. 

 In the postpharyngeal region, however, the size of the head in- 

 creases and the length of the neck decreases until in the pos- 

 terior half of the postpharyngeal region the head is about as 

 large as in anterior pieces and the eyes appear in the plane of 

 the cut. Both species of Planaria are similar in this respect. 

 Figs. 8, 9 and 10, a, b, c, illustrate the case under consideration. 

 Fig. 8 indicates the regions from which the pieces a, b and c were 

 taken ; in Fig. 9, a, b, c, the difference in size of the head and 

 length of the " neck " in the new tissue is shown at an early 



