September 2 1, 1905J 



NA TURE 



521 



■stance is made up of minute granules or bioblasts, which 

 are the real vital units or elementary organism, embedded 

 in a homogeneous substance, the non-living matter. Cells 

 are formed by a combination of these units of a lower 

 order, and are therefore individuals or units of the second 

 order. 



At about the same time Butschli brought forward his 

 celebrated hypothesis of the froth or alveolar structure of 

 cytoplasm. This was based upon an extensive series of 

 observations upon both living and dead cells as well as 

 upon froths or foams made by mi.xing salts of various 

 kinds with oil and then placing small particles of the oily 

 mixtures so obtained in water. 



Butschli compares the structure of cytoplasm to that 

 of a fine froth, and considers that much of the granular, 

 and network or fibrillar structure can be referred to the 

 optical appearances presented by such a froth. That such 

 structures are visible cannot be doubted by anyone who 

 has examined these froths attentively with the microscope. 

 But that all the fibrillar structures described by Fromman 

 and Flemming, whose observations have often been con- 

 firmed since by competent cytologists, can be referred to 

 a froth structure, cannot, I think, be accepted by anyone 

 who has carefully examined plant cells. 



From the fact that cytoplasm appears homogeneous 

 under certain conditions, and that the foam structure can 

 be so readily produced in it by various means, and further 

 that, as Hardy has shown, the action of certain reagents 

 upon colloids results in the separation of solid particles 

 which become linked together to form a comparatively 

 coarse, solid framework in the form of an open net which 

 holds fluid in its meshes, it is probable that we shall find 

 the foam-structure theory of protoplasm is not tenable. 

 It seems far more in accordance with what we know 

 that we should regard protoplasm as fundamentally a 

 semi-fluid, homogeneous mass, in which, by its own 

 activity, granules, vacuoles, fibrils, &c., can be produced 

 as secondary structures ; and that any special morpho- 

 logical structure which it may possess is beyond the limits 

 of the present resolving powers of the microscope. 



The Structure of the Nucleus. 



From the recent observations of Gregoire and Wygaerts, 

 Berghs, Allen, Mano, and others, it is difficult to arrive 

 at any definite conclusions as to the structure of the 

 nucleus, or as to the changes which take place in it lead- 

 ing to the production of the chromosomes. The resting 

 nucleus seems to possess a very simple organisation. In 

 the living condition it appears to consist merely of a 

 homogeneous ground substance in which is contained a 

 mass of chromatin granules which do not appear to have 

 any particular shape, and one larger granule of a spherical 

 shape, the nucleolus. Sometimes a network or foam 

 structure is visible, but not always ; but here, as in the 

 cytoplasm, it is difficult to be certain of this. It may be 

 that the chromatin is always in the form of an irregular 

 network embedded in the colourless ground substance, and 

 that the granular appearance is due to an optical effect 

 similar to that observed in finely meshed oil-foams. 

 According to Strasburger, Miss Sargant, Farmer and 

 Aloore, Mottier, and others, the nucleus contains an achro- 

 matic network — the linin — in which the chromatin granules 

 are embedded. Mano, Moll, and Sypkens deny the exist- 

 ence of these two substances, and state that the network 

 consists of chromatin only ; while Gregoire and Wygaerts, 

 Allen and Berghs, are inclined to the view that there is 

 a fundamental basis of linin which is impregnated by 

 chromatin ordinarily diffused through its whole substance, 

 but capable of being collected into certain definite regions 

 under certain conditions by which the granular appear- 

 ance is produced. The evidence brought forward in many 

 of the more recent investigations certainly goes to show 

 that the chromatin is not in the form of such definite 

 granules as was at one time supposed ; that they are not 

 so regular in size or outline ; and that it is not easy to 

 differentiate between the chromatin and achromatin con- 

 tents of the nucleus. Staining reactions do not afford a 

 sound clue to their differentiation, for, as Fischer and, 

 more recently, Allen have shown, the differences in stain- 

 ing reactions of the different parts of the nucleus vary 

 according to the strength of the stain, the time it is 



NO. 1873, VOL. 72] 



allowed to act, and the size or thickness of the granules 

 or threads stained. 



Strasburger has suggested that the chromosomes are 

 formed by the fusion of gamosomes (chromatin granules) 

 around gamo-centres into zygosomes (chromosomes), but 

 the changes which take place are probably not so clearly 

 defined as this. What seems clear from the facts we 

 know is that the substance forming the homogeneous 

 chromosomes — the chromatin or nuclein — becomes broken 

 up in the reconstitution of the daughter-nuclei, by vacuo- 

 lation or otherwise, into an irregular network which 

 presents a granular appearance. In this all trace of the 

 original individual chromosomes is in most cases lost, 

 and at the same time one or more deeply staining bodies 

 of a spherical, or nearly spherical shape — the nucleoli — 

 appear in contact with it. 



The Nucleolus and its Function. 



The evidence is steadily accumulating that the nucleolus 

 is intimately concerned in the formation of the chromo- 

 somes, although probably not exclusively concerned in this 

 function. In most cases it appears to form a part of the 

 chromatin network, being connected to it by threads, and 

 generally gives similar reactions to the chromosomes. In 

 some few cases it is described as completely separated 

 from the network by a clear area which is visible both 

 in the living and in the stained condition. The evidence 

 that the nucleolus is concerned in chromosome formation 

 may be summed up as follows : the nucleoli are closely 

 connected or associated with the nuclear network ; as the 

 nuclear network becomes more deeply stained the nucleoli 

 become smaller or lose their capacity for stains ; at the 

 time the chromosomes are being differentiated they are 

 connected to the nucleoli by delicate threads ; the chromo- 

 somes resemble nucleoli in their behaviour towards re- 

 agents and stains ; during the period of sinapsis the 

 nucleoli come into very close relations with the nuclear 

 thread, and as the nucleus gradually passes out of the 

 sinaptic stage the thread stains more deeply ; in the re- 

 constitution of the daughter-nuclei the chromosomes can 

 be seen to fuse together into a more or less irregular 

 mass, out of which the delicate nuclear network and the 

 prominent nucleolus are evolved ; in certain cases all the 

 chromatin appears to be stored up in the nucleolus. 



It has been suggested that the nucleolar substance is a 

 product of excretion of the nucleolus, but there is very 

 little evidence for this view. On the other hand it is 

 very likely, as suggested by Mottier, that the nucleolus 

 contains a store of nutritive material which can be used 

 up for various purposes, both in the nucleus and in the 

 cytoplasm. In some cells a portion of the nucleolar sub- 

 stance is thrown out into the cytoplasm during the division 

 stages, and it is very probable that this may have some 

 important connection with the metabolic activity of the 

 cell at this period. 



Division of the Nucleus in the Spore Mother-cells. 



The divisions of the nucleus which lead immediately to 

 the formation of the spores possess some features which 

 are not found in ordinary vegetative mitosis, and which 

 have an important bearing upon the facts of heredity. 

 The first of these is known as the heterotype, the second 

 as the homotype division. The essential features of the 

 heterotypical division are as follows : — The chromatin net 

 becomes gradually resolved into a more or less continuous 

 spireme. This thread (or threads) contracts into an 

 irregular mass around the nucleolus, a phenomenon which 

 was first discovered by Moore, and to which he gave the 

 name of " sinapsis." Some observers regard this con- 

 traction as caused by reagents ; but since it has been 

 observed in the living condition by Miss Sargant and 

 others, it is probably a definite and normal stage in the 

 division. It is concerned with some very pronounced 

 changes which take place at this time in the nucleus. 

 The nuclear thread becomes more prominent, stains more 

 deeply and exhibits a double row of granules which gives 

 it the appearance of a double thread. This has been 

 variously interpreted by different investigators : Miss 

 Sargant, Farmer and Moore, and many others consider 

 that it is due to a longitudinal splitting of the thread ; 

 Dixon, Gregoire, Berghs, and Allen consider it as in- 

 dicating a close appro.ximation of separate loops of the 



