48 RE GENERA TION 



and new tail. This is best shown when the piece includes the old 

 pharynx (Fig. 22, C). The pharynx is also shifted, so that its anterior 

 end points towards the side at which the new head lies, and its pos- 

 terior end towards the new tail. The result is that a new symmetri- 

 cal worm is formed, as shown by the series of figures in Fig. 22, C. 

 In Planaria maculata the changes take place largely in the old part, 

 and the old material extends throughout the entire length of the new 

 worm. In Planaria lugubris the change takes place largely in the 

 new parts (Fig. 22, B\ The general method in the latter species by 

 which the symmetry is attained can be best shown by cutting the 

 worm in two by an oblique cut just in front of the genital pore (Fig. 

 22, A). The posterior piece produces a new head at the side, and a 

 new pharynx appears along the border between the new and the old 

 parts, as shown in these figures. Its posterior end touches the middle 

 line of the old part, and from this point it extends obliquely across 

 the new tissue towards the middle of the new head. As regenera- 

 tion goes on the new head is carried farther forward, it becomes 

 larger, and the main region of new growth is found to be, in the fig- 

 ure, to the left side of the new part. As a result of these changes the 

 new head turns forward, and comes to lie nearer the middle line of 

 the old part. The pharynx is also turned more forward, and finally, 

 as the new parts enlarge, the symmetrical form is produced. The 

 internal factors that are involved in the development of these oblique 

 pieces are very difficult to analyze. The position of the new head 

 and tail at one side of the cut-edge is the most difficult phenomenon 

 of all to explain. We may, I think, safely regard the first new mate- 

 rial that is proliferated along the cut-edge as totipotent, and our spe- 

 cial problem resolves itself into discovering what factor or factors 

 determine that the new head is to form at the most anterior end of 

 the new material, and the new tail at the most posterior end. If we 

 assume that the result is in some way connected with the influence 

 of the old part on the new, and that this influence is of such a sort 

 that the more anterior part of the old tissue determines that one side 

 of the head must be at the most anterior edge, we have at least a 

 formal explanation of the position of the head at the side. Given the 

 position of the new head fixed at one side, its breadth will be determined 

 by the maximum breadth possible for the formation of a new head. 

 This is also in part an assumption, but it has at least certain general 

 facts of observation in its favor. The oblique position of the new 

 head is the result of its symmetrical development in the new material 

 in the same way that the position of the tail of the fish or of the tad- 

 pole is the result of .the symmetrical formation of the new tail on the 

 oblique surface. The subsequent changes, by means of which a sym- 

 metrical worm is developed, are the result of different rates of growth 



