804 MORPHOGENESIS 



new contractile vacuole develops from the long vacuolar feeding canal 

 near the posterior end of an anterior body section, but never at the for- 

 wsLtd end of a posterior section. The new vacuole appears only at the 

 posterior end of a piece cut from the mid-body region. After a deep 

 transverse cut in the mid-region, a vacuole appears near the cut end of 

 the anterior half; but when the wound closes the new vacuole disap- 

 pears, as the posterior part of the feeding canal fuses with it. In long 

 anterior pieces, in short anterior and posterior pieces, and in cut-out 

 sections, a new mouth develops at the appropriate position; those cut 

 in such a manner that the old mouth occupies an odd position will re- 

 generate a new mouth in the correct position, and the old one is re- 

 sorbed (Fig. 184). The specificity and location of these regenerated 

 structures in Spirostomum are attributed to the determining action of 

 the physiological gradients. 



/ 

 Regeneration in Colonial Forms 



The foregoing pages have dealt with different aspects of regeneration 

 as it occurs in wounded cells or in their dismembered parts. Whereas this 

 phase of morphogenetic investigation revolves about the cell, its funda- 

 mental organization, or the relations between different regions of the 

 same cell, an equally fruitful line of inquiry concerns the relationships 

 of one cell to another in true or temporarily colonial Protozoa. 



Twenty-five years ago Runyan and Torrey (1914) became interested 

 in the determination problem in VorticeUa sp. when they discovered 

 that, after division, the migratory cell always forms from the lateral 

 daughter. The cleavage plane in this peritrich coincides with the longi- 

 tudinal cell axis. But instead of bisecting the aboral or attached end of 

 the constricting cell, the plane deviates from the mid-line enough to 

 disrupt the stalk connections of one (the lateral) daughter. This is the 

 presumptive migrant or "ciliospore." The other cell retains its continuity 

 with the stalk and remains behind to repeat the division process at a 

 later time. Metamorphosis of the lateral cell does not begin until the 

 protoplasmic junction between the two cells is reduced to a small thread. 

 These observations led Runyan and Torrey to suppose that the posterior 

 girdlet of cilia, the scopula, and other features of the migrant, appear 

 only after the cell is physiologically isolated from the stalk; and, further- 

 more, that such isolation does not exist until the organic connection 



