524 



NATURE 



[September 21, 1905 



plast is not known ; but that starch can be formed, in- 

 dependently of chlorophyll, in the leucoplasts and in the 

 ordinary chloroplasts directly from sugar and other organic 

 solutions in the dark seems to indicate that the two are 

 not necessarily connected. 



The colourless stroma of the chloroplast gives a distmct 

 and pronounced reaction for phosphorus when treated 

 according to Macallum's method. It resembles, therefore, 

 in this respect the nuclein constituent of the nucleus. 

 What the exact significance of the presence of phosphorus 

 in the chloroplast may be I do not know, but it is ex- 

 tremely interesting to find that in an organ in which a 

 high degree of metabolic activity is always found a sub- 

 stance should be present which is akin to the highly 

 organised nuclear constituents. It suggests an interesting 

 comparison with those plants in which a special starch- 

 forming organ, the pyrenoid, is differentiated. 



The Centrosomes and Centrosphercs. 



A vast literature has grown up in connection with the 

 structure and function of these bodies because of the 

 special importance which has been attached to them as 

 the originators of the process of nuclear division and of 

 the formation of the spindle, and because of the important 

 part which it is assumed they play in the phenomena of 

 fertilisation. 



Their very general occurrence in animal cells and their 

 prominence in the reproductive processes led plant cyto- 

 logists to predict that they would be found to occur also 

 in plant cells. But their prediction has not been fulfilled. 

 They are frequently found among the Thallophytes and 

 Bryophytes, but in the higher plants the evidence is steadily 

 accumulating against them, and such structures as have 

 been described by Guignard and others are held to be 

 based upon a misinterpretation of the facts observed. 



Where the centrosome exists it consists of a deeply 

 stained granule or group of granules surrounded by radi- 

 ating fibres. In some cases, as in the Basidiomycetes, 

 the centrosomes only become definitely visible as minute 

 dots at the poles of the spindle, and are not visible until 

 this is completely or nearly completely formed. In other 

 cases, as in Dictyota (Mottier), Ascomycetes (Harper), 

 the centrosomes with their radiations are clearly visible 

 at two opposite sides of the nucleus in the resting stage, 

 and are in close contact with the nuclear membrane. In 

 the Ascus, Harper has shown that the centrosome is in 

 close contact, not only with the nuclear membrane, but 

 also with the chromatin net, and it seems probable that 

 there may be a connection between them. The spindle 

 fibres are formed both in Dictyota and in the Ascus in 

 the nuclear cavity before the nuclear wall breaks down. 

 In the division of the daughter-nuclei the centrosome which 

 is carried over with each daughter-nucleus appears to 

 divide — but this is not certain — to give two new centro- 

 somes for the formation of the new spindle figure. 



Experiments on the Production of Artificial Asters. 



There are two main views as to the nature of the 

 spindle and astral fibres : (i) that they represent a definite 

 morphological differentiation of the cytoplasm which 

 possesses in itself the power of forming these fibres ; 

 (2) that they are formed out of the cytoplasm by some 

 modification of its structure or arrangement of its parts, 

 or by the precipitation or condensation of some of its 

 constituents. 



The aggregation of granules into radiating fibrils can 

 be imitated artificially by allowing a drop of alcohol or 

 turpentine to fall upon smoked glass. If the drop is 

 allowed to fall from a good height, we get the artificial 

 centrosomes with radiations first described by Henking ; 

 these are due mainly to the splash of the drop and its 

 breaking-up into small particles which radiate outwards, 

 carrying portions of the smoke film with them. If the 

 drop is allowed to fall more gently, so that it does not 

 splash, its first effect is to produce a clearly circumscribed 

 circular ring, and then, by slowly spreading outwards, 

 to produce an aggregation of the smoke particles into 

 fibrils which more nearly represent the appearances pro- 

 duced in cytoplasm than do Henking's splashes. 



By careful manipulation we can get in this way re- 

 presentations of the centrosome or centrospljere, or even 



NO. 1873, VOL. 72] 



the radiations around the nucleus. If the edge of thf 

 alcohol or turpentine be carefully examined under the 

 microscope as it is slowly spreading outwards, a violent 

 motion of the smoke particles will be observed as soon 

 as the liquid comes into contact with them, and as the- 

 liquid passes on these particles settle down into definite 

 continuous fibrils, which go on growing as the liquid 

 continues to spread. 



Fischer has described the formation of artificial asters 

 by two methods : (i) If pith is injected with proteid and 

 then fixed, asters are found around small particles of 

 foreign matter in the proteid. (2) If a small granule of 

 corrosive sublimate or a drop of osmic acid be brought 

 into a proteid solution radiating striae are formed in it 

 by precipitation. He suggests that the centrosome is 

 formed by the precipitation of albuminous substances in 

 living ceils by the excretion of nucleic acid from the 

 nucleus, and that, as in (i), artificial radiations are formed 

 around it by the action of the fixing reagents ; or possibly 

 by the fixative action of the nucleic acid itself. Or the- 

 centrosome itself may produce them, as in (2), by acting, 

 as the precipitating agent, just as corrosive sublimate or 

 osmic acid. Mr. Jenkinson has recently described some 

 interesting experiments on the artificial production of 

 asters, and comes to the conclusion that osmotic pressure 

 and surface tension are probably concerned in the form- 

 ation of these structures in the living cell. The centro- 

 some may be a body capable of withdrawing water from 

 the cytoplasm, of swelling up and dissolving in the water 

 so absorbed, and then giving off radial outgrowths which 

 precipitate the proteids of the cell, and so form astra! 

 rays ; or the centrosome may undergo decomposition, or 

 mav secrete a ferment which would have the same effect 

 upon the cytoplasm. 



The Blcpharoplast. 



The blcpharoplast is a special organ associated with 

 the formation of the cilia in motile spermatozoids and 

 zoospores. It consists of a centrosome-like granule, often 

 surrounded by radiations. It appears inside the cell in 

 close relation to the nucleus, or sometimes at the periphery 

 of the cell. In Polytoma the two cilia thus arise from a 

 granule (blcpharoplast) at the extremity of the cell. In 

 .'Edogonium the blepharoplast arises, according to Stras- 

 burger, in the plasma membrane. Strasburger considers 

 them as kinoplasmic in nature, and thus brings them into 

 relation with his other kinoplasmic structures, the centro- 

 some and spindle. 



Some authors consider that the blepharoplast is a true 

 centrosome, or homologous with a centrosome. It has 

 not, however, been conclusively shown that it at any 

 period in its history performs the function of a centro- 

 some, or that it is derived from one. Further, in many 

 of these plants, if not all, there are no centrosomes at any 

 stage in their life-history. 



On the whole the evidence is distinctly against the view- 

 that the blepharoplast is genetically connected with the 

 centrosome. It is more in accordance with the present 

 state of our knowledge to consider the blepharoplasts as 

 special structures which arise de novo in the cell for the 

 special function of cilia formation. 



The Coenocentrum and its Function. 

 In the oogonia of some fungi there appears at an early 

 stage in the development of the obsphere a dense granular, 

 deeply stainable substance, the function of which is un- 

 known. It appears in the centre of the cell, and was first 

 discovered in the obsphere of Cystoptis (Albugo) Candida. 

 It is probably formed by an accumulation of stainable 

 granules or microsomes. It disappears soon after fertil- 

 isation takes place, and is therefore not a permanent 

 organ of the cell. Shortly after its appearance one of 

 the nuclei out of the large number irregularly scattered 

 through the oogonium conies into contact with it, and 

 gradually becomes more or less embedded in it. All the 

 other nuclei pass to the periplasm, leaving this single 

 nucleus as the nucleus of the ovum. The fertilising tube 

 which contains the male nucleus also grows towards it, 

 and comes close to it to discharge the male nucleus upon 

 it. This indicates that it may exert in some way or other 

 an attraction, first upon the female nucleus, and secondly- 



