CERTAIN GENERAL CHARACTERISTICS 25 



show four stages in the cell division of Euglena, as described by Ratcliffe 

 (1927). The nucleus approaches the anterior end before division, and the 

 behavior of the blepharoplasts and the progress of cytoplasmic division 

 from the anterior end posteriorly indicate that physiological conditions in 

 the anterior region of the cell differ in some way from those in more poste- 

 rior parts. In certain choanoflagellates division progresses posteriorly in 

 the cell body and anteriorly in the collar. Division in the cystoflagellate 

 Nodiluca progresses over the body from the mouth region, which is un- 

 doubtedly to be regarded as apical. Division of certain trichomonad 

 flagellates progresses from the anterior end posteriorly (Kofoid and Swezy, 

 191 5). In Noctiliica and the trichomonads the nucleus is situated in the 

 anterior end, but in others which show the same course of the division fur- 

 row from anterior to posterior this is not the case. In the two species of 

 Spirotrichonympha recently described by Cleveland (1938) the reconstitu- 

 tion of the highly differentiated morphological structure in connection 

 with fission progresses basipetally from the apical end of the new individ- 

 ual, which in one species is half the apical end of the original animal and 

 in the other is a new apical region at the original posterior end. Longitu- 

 dinal division also occurs in the stalked ciliate protozoa such as Vorticella, 

 Carchesium, Zoothamnium, etc., the division furrow appearing first at the 

 distal end of the zooid and progressing proximally. It is well known that 

 in Stentor and various other forms the peristome region develops first in 

 fission; development of the new gullet in fission of the heterotrichous cili- 

 ate Bursaria progresses from the anterior end (Lund, 1917). 



Indications of a longitudinal differential also appear in free-swimming 

 ciliates which divide transversely, though these are usually less definite 

 than in flagellates and sessile ciliates. When the temperature of a Parame- 

 cium culture is raised from i8°-2 2° C. to 26°-30° C, the anterior member 

 of a pair resulting from fission undergoes the following division earlier 

 than the posterior member; but when the temperature is lowered from 

 18° 22° C. to 13°-! 7° C, the posterior member divides first. Even in the 

 third generation at high and low temperatures some indications of this 

 difference persist (W. Petersen, 1927). The author interprets these results 

 as due to a difference in physiological condition in anterior and posterior 

 regions which results in a differential susceptibility to high and low tem- 

 peratures, the high temperature accelerating, the low temperature de- 

 pressing, the anterior more than the posterior member. De Garis (1928) 

 followed division rates in five lines of Paramecium with selection for ante- 

 rior and posterior origin. In three lines the anterior selections showed a 



