August 4, 1911] 



SCIENCE 



131 



lieve that the different types are referable 

 to one common generalized type which 

 Biitsehli described as alveolar in structure. 

 A simple example of such modification of 

 the alveolar into denser plasm can be 

 easily demonstrated in the protruding 

 pseudopodium of Amoeba proteus. Here 

 the endoplasmic alveoli become drawn out 

 into ellipsoidal forms, the alveolar walls 

 come together and fuse, forming the char- 

 acteristic denser ectoplasm. Another good 

 example of the same metamorphosis may 

 be seen in the formation of the temporary 

 membrane which appears between the ecto- 

 plasm and the endoplasm of ActinospJice- 

 riiim eichhornii. 



Second, as to nuclei. The study of pro- 

 tozoan nuclei has taught us that a definite, 

 formed nucleus is not essential for cell 

 life. There are many cases amongst the 

 protozoa where there is no morphological 

 nucleus, but the functions of this organ- 

 oid of the cell are presumably performed 

 by fragments of chromatin distributed 

 throughout the protoplasm. Such is the 

 case, for example, in Dileptus gigas, where 

 each granule at cell division elongates and 

 divides. When formed nuclei are present 

 they are provided with a firm and thick 

 membrane which does not disappear dur- 

 ing division as in nuclei of higher animals 

 and plants. The chromatin also, is not ar- 

 ranged in a reticulum as in higher forms, 

 but is usually massed in one or several 

 solid bodies termed karyosomes. These 

 have often been called chromosomes, but 

 such use of the term is incorrect, for these 

 karyosomes in many cases break down into 

 finer granules which are secondarily fused 

 into elements strictly homologous with 

 chromosomes of higher forms. In the 

 protozoa therefore we have abundant ma- 

 terial for working out a possible evolution 

 of these important elements of higher cells, 

 from generalized conditions of the para- 



sitic amcgbte to the formation of primitive 

 chromosomes in Noctiluca or Paramecium. 

 In such primitive forms the number of 

 chromosomes is always greater than in 

 metazoa, more than two hundred having 

 been counted in Paramecium caudatum. 



Third, as to the centrosome. Cytological 

 study of protozoa gives much more direct 

 evidence of the function of this organ of 

 the cell than does its study in egg or tissue 

 cells. In protozoa it is undoubtedly a 

 kinetic center of the cell in the sense of 

 being the central organ in different types of 

 movements. Many types of Heliozoa, such 

 as AcantJiocystis or Spharastrmn, have a 

 definite central granule in the resting cell. 

 At division periods this divides and forms 

 a spindle; the nucleus is drawn into the 

 nuclear plate and connected by fibers with 

 the divided centrosome, and the outcome is 

 a typical karyokinetic figure. After di- 

 vision the spindle fibers and astral rays 

 grow out from the central granule to form 

 the axial filaments of the actinopodia, 

 which in some species of Acanthocystis 

 and Artodiscus have a vigorous springing 

 movement. In DimorpJm both actinopodia 

 and flagella are present and, both having 

 the same origin, we are led to the con- 

 clusion that flagella, in this case at least, 

 are little more than naked axial filaments. 

 Similarly, in various types of flagellates, 

 e. g., Trypanosoma, Eerpetomonas, Cri- 

 thidia, etc., the flagellum forms by out- 

 growth from the centrosome thus proving- 

 the intimate connection between the loco- 

 motor apparatus of the organism and its; 

 centrosome. 



In many cases this kinetic center is in- 

 side of the nucleus — giving what Boveri 

 called the centronucleus type of nucleus. 

 In such eases the axial filaments of Helio- 

 zoa abut against the nuclear membrane 

 (e. g., in Actinophrys, Actinosphcerium, 

 Camptonema, etc.), and during division 



