326 FAMILY: TRYPANOSOMID^ 



He regards the deeply staining band as the axial filament, here well 

 developed, while the faintly staining line passing to the opposite extremity, 

 and not always present, as representing the axoneme. Of the two views, 

 it seems to the writer that the first one is correct, otherwise one must 

 assume that this particular form is totally different from all other flagel- 

 lates of the group. The writer (1913a) studied these forms in the house- 

 fly, and could find no evidence to support Alexeiefl's view. 



NUCLEUS. — The nucleus, which can only be satisfactorily studied in 

 specimens which have been prepared without drying, generally consists of 

 a nuclear membrane enclosing a clear space, at the centre of which is a 

 karyosome which stains deeply with chromatin stains. In some of the 

 larger trypanosomes, radiating fibres — the nuclear meshwork — can be 

 seen connecting the karyosome with the nuclear membrane (Fig. 247). 

 The latter is clearly seen as a definite structure in the largest trypano- 

 somes. In the smaller forms it may be difficult to distinguish from the 

 surrounding cytoplasm, the nucleus appearing as a clear space in the 

 cytoplasm with the karyosome at its centre. In such cases it is probably 

 safe to assume that a very fine membrane is present, for the clear space 

 persists in nuclear division, during which it becomes elongated and finally 

 divided. In the nuclei of the larger trypanosomes, in addition to the 

 central karyosome, smaller granules, apparently of a chromatin nature, 

 are sometimes present on the inner surface of the nuclear membrane or 

 even distributed upon the nuclear meshwork. Occasionally, in place of 

 the single karyosome, several comparatively large chromatic bodies are 

 present. The karyosome usually stains uniformly and intensely, but in the 

 larger forms several more deeply staining areas may be present, suggesting, 

 as Robertson (1909a) has pointed out for Trypanosoma raicB, that it may be 

 made up of two substances, the chromatin proper and a plastin material. 

 Sometimes a more deeply staining granule has been distinguished at the 

 centre of the karyosome, and from what occurs in nuclear division this 

 granule has been interpreted as a centriole or intranuclear centrosome. 

 Nieschulz (1922a) has noted the presence of a small granule on the nuclear 

 membrane of the crithidia forms which appear in cultures of bird trypano- 

 somes (Fig. 154, 9). It was not present in the trypanosome forms. It 

 miist be remembered, however, that the appearance of the karyosome 

 after such stains as iron hsematoxylin varies considerably with the degree 

 of extraction of the stain. 



In nuclear division, as usually seen in wet fixed films, there is an elonga- 

 tion of the nucleus associated with an elongation of the karyosome (Fig. 

 156, i). The latter finally becomes constricted at its centre and divided into 

 two daughter karyosomes. The nuclear membrane then becomes constricted 

 between these, and two nuclei similar to the original one are produced. 



