120 BIOLOGY OF THE PROTOZOA 



formality, the nucleus meantime assuming an indefinite division 

 figure. More frequently, however, the chromomeres are suspended 

 between an endosome and the nuclear membrane, as in Thylaco- 

 vionas comjyressa, Eidrepfia viridis, Eimeria scJmbergi, Oxyrrhis 

 marina, or various species of Trypanosoma. In some of these, at 

 division the chromomeres appear to form a nuclear plate, and 

 are distributed in equal groups to the daughter nuclei (Fig. 62). 

 In Euglena riridw, which usually is placed in this group, the chromo- 

 meres according to TschenzofT (1916) are derived from a chromatin 

 reticulum and pass through a skein stage before forming a broad 

 nuclear plate in which each is longitudinally divided. In Oxyrrhis 

 marina the chromomeres unite to form linear aggregates which 

 divide longitudinally (Hall, 1925). (Fig. 43, p. 88). 



In a final group of types of nuclear division figures either from 

 massive or vesicular nuclei, the chromomeres are derived from the 

 fragmentation of endosomes or from a chromatin reticulum. The 

 common feature in this large group is the fact that these chromomeres 

 unite secondarily to form definite chromatin bodies which satisfy, 

 in part at least, the definition of chromosome as given above. 

 These chromosomes are di\ided equally, one-half going to each 

 pole of the division figure. In some cases it is obvious that their 

 division is longitudinal, but in the majority of cases it cannot be 

 ascertained with assurance whether their division is longitudinal, 

 or transverse. Nuclear figures of this general type may be divided 

 into two groups, in one of which the chromosomes are too numerous 

 to permit of decision as to their constant number, and the second 

 comprising forms in which the chromosomes are constant in number 

 and in some of which this number is reduced to one-half at meiosis. 

 In the first of these groups we would include types like Euglypha 

 almolata, the various species of Paramecium, Noctiluca miliaris and 

 Oxyrrhis marina. In the second group we would place such forms 

 as Actinophrys sol, Aggregata eherthi, Trichomonas and allied 

 flagellates, Trichonympha and related forms, and the majority of 

 ciliates in which the maturation processes are known. 



In Englypha alveolata the chromatin of the vesicular nucleus is 

 distributed throughout the resting nucleus. During the early 

 division stages the chromomeres are rearranged in rods or fibrils 

 which form a more or less definite skein within the nucleus; this 

 skein fragments into a large number of chromosomes which are 

 longitudinally divided according to Schewiakoft*. A more aberrant 

 history is followed by the chromatin of the nuclei of various species 

 of Paramecivm. In Paramecium caudatum the micronucleus belongs 

 to the massive type, and there is no satisfactory account of the 

 origin of chromosomes in vegetative division. The micronucleus 

 becomes much larger, however, in preparation for the first matura- 

 tion division, when, according to Calkins and Cull (1907), the com- 



