DYNAMICS OF MORPHOGENESIS 127 



at different temperatures certain points concerning the gradient 

 appear. 



Figure 33 shows the characteristic result after eight days at a 

 temperature of 26° to 28°. The posterior outgrowth of new tissue 

 is long, the new intestinal branches have already extended to its 

 posterior end, the pharynx is advanced in development and the 

 change in proportion is marked. The pieces have decreased con- 

 siderably in size because of their high rate of metabolism and the 

 absence of food. All the high temperature pieces are essentially 

 like this. 



Figure 34 represents a normal whole after eleven days at a 

 temperature of 20°. The outgrowth of new tissue is much shorter 

 than in figure 33 and the posterior intestinal branches have not 

 developed as far into the new posterior end, but the pharynx is 

 about the same size in the two cases. In other wOrds the extreme 

 posterior region of the body is less highly differentiated in figure 

 34 than in figure 33 : its development has been retarded to a greater 

 extent, or has ceased at a slightly earlier stage, while farther 

 anteriorly, i.e., in the pharyngeal region, the development is 

 much the same in the two cases. 



In figure 35 a normal whole after thirty-six days at 10° is shown. 

 The posterior new tissue is short and blunt and the posterior 

 intestinal branches have not penetrated it at all : its development 

 as a tail has ceased at a much earlier stage than in the preceding 

 cases. In this case the development of the pharynx also ceases 

 at a much earlier stage than at higher temperatures. It remains 

 small and short and apparently does not attain any considerable 

 degree of motility. H^re, too, the change in proportion is practi- 

 cally absent because the locomotion of the piece is slow and the 

 new tail scarcely functions at all as an organ of attachment 

 (Child, '09). In fact, it is perhaps a question whether the pos- 

 terior outgrowth in these pieces can be called a tail or not. Since 

 it is impossible to determine just where it ceases to be a tail, I 

 have called all cases wholes in which a pharynx is present and the 

 formation of the posterior intestinal branches has begun. It is 

 evident at any rate that in such pieces the morphogenetic proc- 

 esses in the postpharyngeal region are more completely inhibited 

 or cease at an earlier stage than in the pharyngeal region. 



