THE STRUCTURE OF CELLS 



true cytoplasmic origin, however much it may have become modi- 

 fied or altered under more special conditions. The chromatiri in 

 these primitive nuclei is often aggregated into clumps as in Actino- 

 sphaerium, Noctiluca, Coccidium, etc., or even concentrated into one 

 mass as in Actinophrysand Spirogyra. These masses have been termed 

 nucleoli by some writers, but recent investigations tend to show that 

 they really represent composite structures which contain other 

 substances in addition to chromatin. In coccidia, for example, 

 Schaudinn and others have shown that, although the chromosomes 

 are derived from them, there exists over and above the chromatin 

 a considerable mass of substance which is left behind after its exit, 

 and much the same is seen in Amoeba hyalina. Possibly their 

 analogues may be sought in certain of the so-called pseudo-nucleoli 

 of the higher organisms, or in those occurring in the nuclei of 

 Spirogyra, where it has been repeatedly asserted that the chromo- 

 somes originate from the nucleolus. As regards the origin 

 generally of the chromosomes and of the peculiar features exhibited 

 by them during mitosis, there seems but little doubt that they have 

 arisen through stages like those seen in tetramitus and chilomonas, 

 in which the distribution of isolated chromatin granules can be 

 followed. The granules first become aggregated into definite tracts,, 

 and these form the primordia of the chromosomes themselves. The 

 actual stages passed through are obscure, and even allied species 

 exhibit considerable differences amongst themselves. Thus in the 

 Amoeba, amitosis seems regularly to occur in Amoeba brevipes and 

 A. polypodia, and also in A. crystalligera ; but in A. hyalina, accord- 

 ing to Dangeard, the chromatin separates from a central body and 

 is differentiated to form chromosomes, whilst the remainder of the 



body gives rise to a rudi- 

 mentary spindle which is 

 entirely intra-nuclear. In 

 A. binucleata the two nuclei 

 divide simultaneously in a 

 mitotic manner, and the 

 same is true of the colonies 

 of the amoeba-like myxo- 

 mycetes. When a plas- 

 modium is about to form 

 spores, it may be found 

 with nuclei showing typical 



Flo 19 karyokinetic figures, all the 



nuclei being in the same 

 phase. 



A remarkable dimor- 

 phism occurs in the nuclei of Paramoeba eilhardi, in which Schaudinn 

 describes one of them as resembling a normal amoeban nucleus, 



B 



Amoeba binucleata. A, with resting nuclei. B, 

 the two nuclei in the aster stage of mitosis. (After 

 Dangeard.) 



