58 PROTOZOAN NUCLEUS 



ground for calling it a centriole. Karyosomes do not always stain homo- 

 geneously, and may have a granular structure, while the appearance of 

 a central granule may be merely the result of irregular extraction of stain. 

 Before deciding as to the centriole nature of a granule, it is necessary to 

 trace its division and the separation of the two daughter centrioles, and 

 to observe the actual centrodesmose uniting them. Furthermore, when 

 spindle fibres are developed for mitotic division, the daughter centrioles. 

 will occupy the poles of the spindle. Such appearances must not be of 

 rare occurrence, but must be detected in the majority of dividing nuclei. 

 When a large number of dividing nuclei of any Protozoon such as an 

 amoeba are examined, it is a relatively easy matter to find isolated examples 

 of spindles which have granules at their poles, though the majority of 

 spindles may not show them. The occasional presence of such apical 

 granules does not justify the assumption that they are actually centrioles. 

 It is an undoubted fact that definite spindles may be formed within the 

 nuclei of Protozoa without there being any evidence of centrioles at the 

 poles, though it is not difficult for those who desire to see such structures 

 to convince themselves that granules, which they interpret as centrioles, 

 are present. The fact that during the mitotic nuclear division of the cells 

 of higher animals centrosomes are almost always present, and that un- 

 doubted centrosomes occur in some Protozoa, has undoubtedly led to many 

 structures being described as centrioles or centrosomes which have quite 

 another nature. In the present state of our knowledge it is impossible 

 to state that a centrosome or centriole is an essential constituent of all 

 Protozoan nuclei. Nevertheless, it cannot be supposed that all the 

 descriptions which have been given are erroneous. In the division of the 

 nucleus of Dimastig amoeba gruberi, which has a large central karyosome, 

 the latter structure elongates and becomes dumb-bell-shaped, and finally 

 divided into two parts. As these separate, they remain connected by 

 a fibre which can be shown in many cases to unite two granules which occur 

 in the two daughter karyosomes (Figs. 61, 4, and 120, 12). AVhether such 

 granules are to be regarded as intranuclear centrosomes (centrioles) is a 

 question more difficult to decide. 



The true structure of a nucleus is that which it possesses in the normal 

 living cell. Fixing fluids and other reagents may considerably alter its 

 appearance, so that the greatest care has to be exercised in the interpreta- 

 tion of the structures seen in fixed material. Experience has shown that 

 certain fixing fluids, stains, and reagents produce better results than 

 others, and are reliable in giving accurate pictures of the true structure. 

 Nevertheless, the literature dealing with the Protozoa, especially the blood 

 parasites which have been largely studied in dried films, is full of erroneous 

 descriptions of nuclei. The dry blood film stained by Romanowsky stains, 



