1919] Rhodes: Binary Fission in Collodictyon triciliatum Carter 253 



might be extended to the majority of the simpler Protista, but not 

 necessarily to all. For in this group nature has her experimental 

 laboratory, and we would expect to find discards. "We may well 

 believe that the mechanism found in sexual plants and animals is an 

 extremely modified and specialized adaptation of a much simpler, more 

 fundamental, but thoroughly satisfactory type, characteristic of most 

 binary fission as found today. So much at least we may assume. 



Parthenogenesis might be pointed to as a reversion to such a primi- 

 tive type and such may be the case ; but a study of this phenomenon 

 points rather to its being a still further specialization and adaptation, 

 based upon advanced sexual phenomena or their suspension. If such be 

 the case, we need not look here for the simpler type mechanism of 

 inheritance characteristic of binary fission. Some do regard it as 

 such, however, and such an interpretation naturally leads to the con- 

 clusion that sex is a universal phenomenon. Minchin (1912, p. 130) 

 makes the generalization that sex is "of universal occurrence in all 

 truly cellular organisms. ' ' This attitude does not seem to accord, nor 

 can it be satisfactorily harmonized with facts as understood today. 

 Coulter (1914) would refute such a view, at least in algae. In the 

 simpler flagellates a satisfactory example of syngamy has yet to be 

 found. Dobell's (1908) life cycle of Copromonas has not been veri- 

 fied. What he figures as maturation phenomena may be well explained 

 by comparing his figures with the differential division of the karyo- 

 some in Collodictyon. Still variation and evolution are characteristic 

 of flagellates. In fact, it is back to the flagellates that the origin of 

 higher plants and animals is traced by the large majority of biologists. 



It is little that Collodictyon adds to this much discussed subject. 

 There is a mitotic figure, a mechanism which may well be interpreted 

 as a distributor of hereditary characters. Chromosomes are present. 

 The actual number of these in Collodictyon, as in most Protozoa, is 

 very difficult to determine. They seem composed for the most part, 

 the most evident part, of chromatin, probably upon an achromatic 

 center or skeletal structure. Such achromatic elements must not be 

 confused with "interzonal or connecting fibers," exposed by the 

 diverging chromosomes (pi. 12, figs. 51-53). The chromosomes in the 

 metaphase split transversely. Such a transverse division is capable 

 of interpretation as a longitudinal split in two ways. Either the 

 chromosomes split longitudinally, separate at one end and finally pull 

 apart at the other end, or, during the precocious splitting of the 

 spireme and final prophase the number of chromosomes is doubled 



