120 C. M. CHILD 



twelve days and figure 20 another piece after eighteen days. 

 Neither of these pieces has formed what can be called a tail but 

 both show a small pharynx and in both a normal head has formed, 

 chiefly by a process of redifferentiation. Intestinal regulation 

 has occurred only so far as the formation of the pharynx has 

 displaced and obliterated parts of the original intestine. 



Figures 21 and 22 show two of the postpharyngeal pieces after 

 twelve days. In figure 21a condition often seen in ether is shown : 

 here the cut surface at the anterior end has not contracted in the 

 usual manner but has remained widely open and a very thin 

 layer of new tissue covers it. In figure 22 the anterior end con- 

 tracted and a small amount of new tissue arose. These two 

 figures represent the extremes of difference in the pieces at this 

 stage. None show either eyes or pharynx. 



Figure 23 shows one of the two pieces which lived long enough 

 to develop eyes. The head, so far as it has developed, is almost 

 wholly the result of redifferentiation and shows no trace of auricles, 

 and the piece contains no pharnyx. Such pieces examined under 

 pressure show almost no trace of intestinal regulation ; the lateral 

 intestinal branches usually meet anteriorly, but no prepharyngeal 

 axial intestine is formed (fig. 23). Regulation is evidently con- 

 fined entirely to the head region and even there is limited chiefly 

 to the nervous system with which the appearance of the eyes is 

 closely connected. 



This series shows particularly well how certain morphogenetic 

 processes can be eliminated while others still continue under the 

 same external conditions. 



Figure 24 shows a postpharyngeal piece from another ether 

 series : here a still more extreme modification of regulation appears. 

 The only externally visible evidences of regulatory morphogene- 

 sis are the minute area of new tissue at the anterior end and the 

 single median optic pigment spot. Examined under pressure, 

 such pieces show small cephalic ganglia more or less fused (Child 

 and McKie, '11). No trace of a pharynx exists and the alimen- 

 tary tract is either similar to that in figure 23 or the two parts may 

 be entirely separate anteriorlj^ 



