GROWTH AND DIVISION 69 



forming the parent organism into two individualities the morpholo- 

 gies of which come to exclude each other and are finally separated 

 by fission. To paraphrase one of the ablest students of Stentor: 

 the situation in ciliates is the reverse of that in metazoa since all 



Stage I . Initial appearance of oral primordium for the pos- 

 terior daughter cell, as a transverse rift in the lateral ectoplasm. 



Stage 2. Primordium enlarges by extending anteriorly with 

 a new curvature to the right. Anlage has a faint glisten in 

 reflected light but no cilia are apparent yet. 



Stage 3. Primordium increases in length and membranellar 

 cilia are visible but not yet grown to their final length. Continued 

 multiplication of striping within curvature of the anlage. 



Stage 4. Primordium grown to nearly its full length and oral 

 cilia are organized into closely-packed membranelles which beat 

 in slow^ metachronal rhythm. Moniliform macronucleus still 

 shows no change. 



Stage 5. An enlargement or etched space appears at posterior 

 end of the primordium, site of the future mouthparts. Anlage 

 now embraces many fine stripes. Macronuclear nodes begin 



fusing. 



Stage 6. Posterior end of membranellar band coils inward 

 sharply to form gullet and cytostome. During this stage severing 

 of stripes begins at each side of anterior end of the primordium 

 and progresses on both sides around the cell to form the fission 

 furrow. Macronucleus fused to a compact mass. Fading or 

 partial dedifferentiation of oral pouch and gullet, begun at stage 

 5, is now at its maximum. 



Stage 7. Primordium migrates posteriorly, its anterior end 

 being cut out of the anterior daughter cell whose stripes heal 

 together at once in a herringbone pattern which will for a long 

 time distinguish proter from opisthe. Gullet and cytostome are 

 now nearly complete and fine striping enclosed by the anlage is 

 being carried forward as the new frontal field. Compact macro- 

 nucleus elongates to rod shape and begins renodulation from the 



ends. 



Stage 8. Oral pouch is now formed as an inpocketing of the 

 frontal field adjacent to the cytostome. Primordium migrates to 

 its definitive position at anterior end of opisthe and body striping 

 becomes parallel to the membranellar band. Rod-shaped macro- 

 nucleus is divided by constriction of the furrow which has nearly 

 separated the daughter cells and formed the new posterior pole 

 of the proter. Mouthparts of proter redefined, membranellar 

 band probably proportionately reduced from length in original 

 cell. Fission products then twist apart. (After Tartar, 1958c.) 



