402 PATTERNS AND PROBLEMS OF DEVELOPMENT 



anterior to it is evident from Figure 137. In Figure 138, a chain of 

 S. tenuicauda, there is even more extensive destruction. After section in 

 the anterior zooid (A'A' of Fig. 138, A) the posterior part of this zooid 

 (i.i.i.i.) and the next two zooids (1.1.1.2. and 1.1.2.) are progressively 

 reduced and destroyed by zooid 1.2.1. (Fig. 138, B-F), and this zooid 

 finally remains as a single individual, fission having occurred posterior 

 to it at fission zone I. (Fig. 138, A). This reduction and destruction of 

 zooids occur only when their heads are in early stages or have been re- 

 moved. Sometimes the headless part of an anterior zooid will reconsti- 

 tute a head rapidly enough to prevent its destruction, though it may 

 undergo some reduction before the head region attains a sufficient domi- 

 nance to prevent it. The farther anterior the level of section in the an- 

 terior zooid, the more frequently does head reconstitution result; that 

 is, the higher the gradient-level of section, the more rapidly does head 

 reconstitution take place and the less likely is obliteration of the zooid 

 gradient and destruction of the zooid to occur. On the other hand, an 

 already visible reconstituting head at a cut end may finally be inhibited 

 and destroyed like the developing head regions of younger zooids. 



In the normal development of the Stenostomum chain each new zooid 

 originates from the posterior region of a zooid already present; conse- 

 quently, in all cases a more advanced head region is anterior to one less 

 advanced. The older anterior component of a pair is evidently able to 

 prevent extension of dominance anteriorly from the younger head pos- 

 terior to it. When a headless zooid is anterior to a head, or an earlier 

 zooid anterior to a more advanced head region, without an older head 

 region still farther anterior, the dominance of the posterior head evidently 

 extends anteriorly and obliterates or perhaps reverses the gradient, and 

 reduction and disintegration follow. 



Whether a zooid is able to maintain itself or is destroyed in these ex- 

 periments probably depends chiefly on the developmental stage of its 

 cephalic ganglia and the extent to which they have attained connection 

 with and have brought about reorganization in, the nervous system pos- 

 terior to them. Until a certain degree of nervous dominance over pos- 

 terior regions is attained by a zooid, it may be destroyed by a more ad- 

 vanced head region posterior to it, provided there is not a more advanced 

 head region at a still more anterior level. If these suggestions are correct, 

 it follows that the dominance of an older head region must still be more 

 or less effective in a zooid developing from its posterior region, at least 

 until the head region or the ganglia of this zooid have attained a certain 



