DYNAMICS OF MORPHOGENESIS 1 1 3 



but in only two pieces (fig. 3) does anything resembling a normal 

 posterior end appear and in these two cases the new outgrowth 

 is still embryonic in character and contains no intestinal branches, 

 while in water the intestinal branches enter the new tissue several 

 days earlier. In all the other pieces the posterior new tissue 

 merely fills the concave cut end and does not grow out beyond 

 the contour of adjoining parts (figs. 4 and 5) and in some cases 

 does little more than close the wound (fig. 6) . 



If such pieces are returned to water the posterior end gradually 

 elongates and becomes more slender, though the new tissue shows 

 no marked further increase, and the whole gradually approaches 

 the characteristic shape of such pieces. If, on the other hand, 

 the pieces remain in alcohol of the same concentration, 1.5 per 

 cent, they show some degree of adjustment or acclimatization to 

 the new environment and regulation may proceed somewhat 

 farther than the stages figured above, but is always retarded or 

 inhibited to a greater extent in the posterior than in the anterior 

 region. 



In the 6-pieces conditions are somewhat different. These 

 pieces {5 to5,fig.l) do not contain the old pharynx and though the 

 posterior part of the old pharyngeal pouch is present it degenerates 

 and the new pharynx forms independently of it. Regulation in 

 these pieces consists essentially in the formation of a new head, a 

 prepharyngeal and a pharyngeal region. 



In water regulation occurs as indicated in figure 7: the new 

 head is large and almost always normal and the new prepharyn- 

 geal region represents about the anterior third of the piece. In 

 1.5 per cent alcohol the head is much smaller (figs. 8 and 9), is 

 often abnormal and there is less new tissue (fig. 9) ; the new pre- 

 pharyngeal region is always shorter than in water (fig. 8), the 

 pharnyx is always small and in the more extreme cases may be 

 entirely absent (fig. 9) . That portion of the piece which rediffer- 

 entiates into the prepharyngeal region always becomes narrower 

 than more posterior regions of the piece (fig. 8) and even in cases 

 where the pharynx is entirely absent a short region posterior to 

 the new head is usually more or less narrowed (fig. 9). This 

 narrowed region is the region where regulation has occurred in 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 13, NO. 1 



