STUDIES ON THE DYNAMICS OF MOEPHOGENESIS 277 



tioned by earlier investigators, but no one thus far has analyzed 

 at all fully the conditions which determine their occurrence. 

 In the following brief description the names adopted serve merely 

 as convenient means of distinction. 



^Normal wholes.'' These are individuals which possess all 

 the visible essential parts of the animal as it occurs in nature 

 viz., elongated body, head with pointed auricles and with two 

 distinct eye spots symmetrically placed, a mouth and pharynx 

 and intestine with one median anterior and two lateral posterior 

 main branches. But such normal wholes may differ widely from 

 each other as regards the course of the regulatory processes 

 which give rise to them and also in shape, proportions, size of 

 head, position of pharynx, intestinal branching, etc. For exam- 

 ple, such a whole may possess a relatively very large head, with 

 new tissue extending back to the level of the auricles, a body 

 tapering posteriorly with a slender posterior outgrowth of new 

 tissue and with the pharynx near the posterior end (fig. 1). Or 

 it may, as in fig. 2, approach the shape and proportions of the 

 animal in nature. Here the head is relatively smaller than in 

 fig. 1, the anterior new tissue may end anterior to the eyes and 

 the pharynx lies near the middle of the body. Or finally the nor- 

 mal whole may show the form of fig. 3, with a small head and 

 with the anterior new tissue extending far posterior to the eyes. 

 In such cases the pharynx usually lies in the middle or anterior 

 to it. 



In figs. 1 and 3 we see two more or less opposed methods of 

 formation of a whole. In the one' (fig. 1) the anterior region is 

 the larger and morphogenesis goes on more rapidly in it, while 

 the posterior end is smaller and develops more slowly : in the other 

 (fig. 3) the anterior region is relatively small and its development 

 is relatively slow as compared with that of the posterior region. 

 The differences between the two methods are also clearly shown 

 by the manner in which the change in shape and the elongation 

 of the pieces occurs. In the cases like fig. 1 elongation is rapid 

 and the posterior region becomes very slender because the head 

 develops early and the animal is very active, moves rapidly and 

 uses only the extreme posterior end for attachment. In the pieces 



