PRIMORDIUM DEVELOPMENT 167 



C. Stage-4 divider transected and posterior fragment dis- 

 carded. In continued ''division" the anterior half of the anlage 

 was shifted to the posterior end of the anterior fragment, later 

 moving forward and forming a crescent of membranelles. After 

 the mouthparts were excised, a regeneration primordium 

 appeared which bridged the interrupting crescent. A nearly 

 normal animal was regenerated (though with an extra tube in the 

 frontal field) either by resorption of the crescent or its incorpora- 

 tion into the new membranellar band. 



D. Island primordia formed when post-oral sector was reversed 

 180°. Anlagen formation on both sides of the patch. Islands of 

 membranelles joined with each other and with the second 

 primordium but complete mouthparts were not formed. After 

 several transformations the specimen eventually became normal. 



E. Capacity for erosion of ectoplasm shown in narrow-loop 

 primordium. Graft of 4 coeruleiis produced one normal, one 

 ring- and one loop-primordium. Enlarged view of latter in 

 second sketch shows "erosion", lifting and buckling of ecto- 

 plasmic striping enclosed within the membranellar band, leading 

 eventually to the separation of a bleb of ectoplasm. Only 



membranelles and oral pouch were formed 



F. Etching of clear band alongside an oral primordium. a: 

 Stage-4 reorganizer (anterior) grafted in tandem with a stage- 1 

 regenerator, b: Anlagen synchronized at stage-5, interrupted by 

 oral remnant of the regenerator and stomatogenesis confined to 

 the posterior primordium. c: Oral remnant either resorbed or 

 incorporated. Clear band to right of primordium, apparently 

 from dissolving of ectoplasmic structures, permits viewing 

 through the interior of the cell with its food vacuoles to the 

 ectoplasm on the far side. An elongated singlet produced which 



soon reorganized with two anlagen. 



G. Dissolving of ectoplasmic striping in presumptive oral 

 region {x) at stage 5 — observed in experimental animals but may be 

 an exaggeration of a normal process preceding oral invagination. 



H. Divider in stage 2 transected through the anlage and 



halves rotated. Essentially there v/as no growth in length of the 



sections of the primordium, as if blocked by abutting stripes. 



Oral differentiation w^as incomplete in both halves, but the 



original mouthparts incurred reorganizational resorption. 



I. Development of a V-shaped primordium. These are 

 formed in addition to the host primordium when a fine stripe 

 sector {x) is implanted into the back of a regenerator. The point 

 of the V invaginates to form a good cytostome and gullet though 

 two membranellar bands are involved. Resorption of subtended 

 portion of old peristome permits entrance of new fine stripes into 

 the frontal field. 



M 



