DYNAMICS OF MORPHOGENESIS 111 



On the other hand, the examination of individual pieces indi- 

 cates the existence of differences of some sort at different levels of 

 each piece. In water under the usual conditions pieces like a 

 resemble figure 2 after eight to ten days of regulation. A large 

 new head is present, a considerable outgrowth of new tissue has 

 appeared at the posterior end and the pharynx has regenerated a 

 new posterior portion and has apparently migrated in the ante- 

 rior direction in the body. 



The appearance of the a-pieces of series 71 after ten days in 

 alcohol 1.5 per cent is indicated in figures 3 and 4, which are typi- 

 cal examples. Figure 3 shows the maximum posterior regenera- 

 tion in the whole set of thirty pieces: in figure 4 the posterior 

 regeneration is less and figures 5 and 6 show still other types of 

 posterior ends. In two pieces only did an outgrowth of new tissue 

 like that in figure 3 occur. The other pieces were all like figures 4, 

 5 and 6. 



The pharynx shows much less regeneration in the alcohol pieces 

 than in water, but its apparent migration in the anteriordirec- 

 tion has not been wholly inhibited. As a matter of fact, however, 

 the apparent migration of the pharynx in these pieces is merely 

 the result of a shortening of the pharynx. The distance from the 

 anterior end of the pharynx to the posterior end of the old tissue 

 in figures 3 and 4 is no greater than the length of the anterior 

 half of the pharnyx originally present in the piece (fig. 1). In 

 water where the apparent migration is more rapid and of greater 

 extent (fig. 2) growth does undoubtedly occur in the region which 

 thus comes to lie posterior to the pharynx. 



The difference in shape in water and alcohol is also striking. 

 In water (fig. 2) the posterior end is slender and tapering and the 

 head region is as broad as any part of the body. In alcohol 

 (figs. 3 to 6) the head is much smaller, and the posterior end is 

 blunt and wider than any other part of the body. 



It is evident then that in alcohol the regulatory processes at the 

 posterior ends of the pieces are much more completely inhibited 

 than those at the anterior ends. For the whole series the results 

 are as follows: of the thirty a-pieces twenty-nine, 96.7 per cent, 

 form heads and eighteen pieces or 60 per cent form normal heads, 



