284 CM. CHILD 



are concerned primarily with certain internal conditions which 

 determine and limit the development of wholes from pieces, 

 and as we shall see, only the uniaxial forms, i. e., groups 1, 2, 3, 

 and 5, enter into this consideration. 



3. The course and results of regulation in relation to the size of 

 the piece and the region of the body represented 



We can obtain some idea of the relation of the factors of size 

 of piece and region of body to the course and results of regulation 

 by comparison of pieces of different size and from different regions 

 of the body. I have found the most satisfactory method to be 

 the comparison of series in which the whole body posterior to 

 the head was cut into two, four, six, eight, etc., pieces of as nearly 

 as possible equal length. When we compare the pieces of the same 

 length from different regions of the body of the same or of different 

 individuals in similar physiological condition, the influence of 

 the factor or region appears, while comparison of pieces of differ- 

 ent length from corresponding regions shows the influence of 

 the size factor.- The results of such comparison are briefly stated 

 in descriptive form with figures in the following paragraphs. 



Halves. Fig. 17, ai and iq. Fig. 18 and 19. Anterior and 

 posterior halves show but little difference in regulatior). From the 

 earliest stages, in which the head is distinguishable as such, up to 

 at least the very late stages, the anterior half (fig. 18) possesses a 

 somewhat larger head than ttie posterior half (fig. 19), and the 

 change in shape and increase in length are also somewhat more rapid 

 in the anterior than in the posterior half. The anterior half of 

 large worms usually contains the pharynx, which at first lies pos- 

 terior to the middle, but gradually approaches it, as other 

 authors have noted, while in the posterior half a new pharynx 

 develops anterior to the middle. 



^Everywhere except in the extreme posterior region of the body pieces of equal 

 length and the full width of the body are approximately equal in size. Toward the 

 posterior end where the body becomes narrower, pieces of the same length may 

 have very different volumes and may contain very different numbers of cells. As 

 a matter of fact, however, other factors render this difference practically negli- 

 gible in the species of Planaria which we are considering at present. 



