MORPHOGENESIS 779 



that conjugating pairs may be cut apart without perceptibly altering the 

 trend of events, which, once started by conjugation stimuh, continue to 

 completion. The regeneration requirements, when superimposed upon 

 the internal readjustments already in progress as a consequence of the 

 sexual processes, appear only to prolong redifferentiation in the ex-con- 

 jugants. Calkins dissected away the apical protoplasmic junction con- 

 taining the migratory pronuclei and thereby eliminated the amphinucleus 

 in both conjugants; nevertheless the resulting cells regenerated complete- 

 ly and with full restoration of their vegetative and reproductive powers. 

 Although cytological details are not supplied in this article, as seen in 

 the living material the reorganization processes without exception fol- 

 lowed the same general sequences as outlined for the normal ex-con- 

 jugants (Calkins, 1919). 



In P. caudatum, anterior or posterior cut-offs made during conjuga- 

 tion result in a large number of fatalities. Those which survive regener- 

 ate slowly and eventually divide (Peebles, 1912). 



Cases of autogamy following the separation of conjugants were re- 

 cently described by Poljansky (1938) for Bursaria. The processes of 

 sexual differentiation continue in the majority of individuals derived 

 from pairs split apart four to six hours after the onset of conjugation. 

 In these instances autogamy supplants heterogamy; the two native pro- 

 nuclei of each cell fuse to produce the amphinucleus, from which the 

 nuclear apparatus of the reorganized cell originates. 



A somewhat different situation prevails in Spathidium. Tests for re- 

 generative ability were made at three different phases of the conjugation 

 process by Moore (1924). The fragments obtained while maturation is 

 in progress, but before the exchange of pronuclei, do not regenerate. 

 Those obtained immediately after fertilization regenerate fully, pro- 

 vided the amphinucleus is included. Ex-conjugant pieces containing only 

 the degenerating macronuclei and the maturation by-products achieve a 

 partial restoration of form, but they subsequently dedifferentiate and 

 never divide. 



Encystment. — Slightly starved pre-cystic individuals of Spathidium 

 spathula are capable of regulation in a fair percentage of cases. The 

 usual consequence of cutting at this stage is immediate dedifferentiation 

 and encystment. Interestingly enough, the external form of a regenerator 

 may be restored before encystment, but nevertheless this cell dedifferenti- 

 ates and then encysts in order to complete the nuclear reorganization 



