48 



CELL 



containing some liquid not protoplasm, and includ- 

 ing salts and other substances in solution. In many 

 Protozoa they are 'food-vacuoles,' formed by the 



Kg. 2. 



A, Embryonic cells from growing point of a root ; B, older cells 

 becoming vacuolated. (After Sachs.) 



bubbles of water engulfed along with the food- 

 particles, round which the protoplasm, shrinking 

 from contact, often forms a definite contour. In 

 other cases they are more permanent, and represent 

 minute reservoirs of secreted substance, cisterns of 

 by-products in the vital manufacture of the cell. 

 Finally they may be seats of special activity, where, 

 perhaps, under the stimulus of irritant waste-pro- 

 ducts, the protoplasm exhibits spasmodic contrac- 

 tions and expansions, and forms the so-called ' con- 

 tractile vacuoles,' which in alternate dilatation and 

 bursting often seem to serve to remove fluid from 

 the living matter to the exterior. 



(c) Nucleus. In the great majority of cells a 

 central body of definite composition and structure 

 is present which appears to be essential to the life 

 and reproduction of the unit-mass. In many cases 

 the nucleus is well concealed, but as more skilful 

 staining has revealed its presence in many cells 

 which used to be described as non-nucleated, it is 

 rash to conclude too certainly as to its absence in 

 any particular case. Thus some of the Monera, 

 which were formerly defined as the simplest of 

 simple animal organisms without even a nucleus, 

 have been shown to possess them, and the line of 

 division separating Protozoa into Monera and 

 Endoplastica has therefore been removed. Fur- 

 thermore, the researches of Gruber have shown that 

 in some of the higher Protozoa (ciliated Infusorians) 

 where the nucleus seems entirely absent, dex- 

 terous staining prove its diffused presence in 

 the form of numerous granules which take on the 

 characteristic nuclear dye. Yet in some cases, 

 such as the young spores of some Protozoa, 

 the greatest care has not yet been successful in 

 proving the presence of the nucleus. In contrast 

 with these cases, many cells exist in which the 

 nucleus is represented not by one, but by many 

 bodies the so-called polynuclear state. A further 

 reserve requires to be made, that it is to a large 

 extent an hypothesis that all such definite central 

 inclosures should be slumped together under the 

 one title of nucleus. It is rather probable that in 

 this, as in other organic structures, we have to do 

 with various degrees of development and definite- 

 ness. 



In the form also of the nucleus numerous modi- 

 fications occur. In the majority of cases, indeed, 

 it is more or less spherical, but it may be elongated, 

 curved, horseshoe-shaped, necklace-like, and even 

 branched. In the young stages of some ova it is 

 like the entire cell, somewhat plastic, and is pulled 

 in and out in amoeboid movements. In special con- 

 ditions, furthermore, the nucleus may exhibit 

 peculiar deformations. It is in fact a peculiarly 

 sensitive and all-important part of the cell, suffering 

 with it in degeneration, changing with it in growth 

 and division. 



In position the nucleus is typically central, where 

 as the presiding genius of the cell it shares and 

 perhaps controls the general protoplasmic life. But 

 it frequently suffers displacement both of a passive 



and active nature. In accordance with the growth 

 of the cell it may occupy a position distinctly 

 nearer one of the poles. Accumulations of fat or 

 mucus may push it passively to the side. Or it may 

 actively change, in response to hidden forces of 

 attraction between it and the surrounding proto- 

 plasm, in the case of some ova exhibiting a peculiar 

 rotation, or else distinctly shifting its ground from 

 the centre towards the periphery. 



Structure. In many cases, as Leydig especially 

 has shown, the nucleus seems to lie in a nest of its 

 own, in a clear space within the surrounding cell- 

 substance. Nor is it in many cases at least de- 

 finitely insulated from the surrounding protoplasm, 

 but is moored to the latter by strands which have 

 intimate relations with both. As of the entire cell, 

 so of the nucleus it must be said that in the great 

 majority of cases it is very far from being homo- 

 geneous. According to Hertwig, Schleicher, Schmitz, 

 Brass, and others, homogeneous nuclei may indeed 

 occur, but if they do they are rare, and it must 

 always be remembered that the nucleus has its 

 history, and may be less complex at one time than 

 it is at another. To Flemming (1882) above all is 

 due the credit of having elucidated the complexity 

 of the nucleus, and the labyrinthine structure to 

 which he showed the clue, and to which Frommann 

 (1867) had many years previously directed special 

 attention, has been studied and restudied by scores 

 of expert histologists during the last six years 

 (1888). While their results disagree abundantly 

 on minor points, two conclusions stand out clearly 

 ( 1 ) that the nucleus has a structure like that of 

 the general cell, consisting of firmer framework 

 and of more fluid intermediate substance, and (2) 

 that apart from detailed difference there is through- 

 out the world of cells a marvellous unity of struc- 

 ture and process, in the nucleus in repose and in 

 the nucleus in action. 



In the nucleus the following parts have to be dis- 

 tinguished : ( 1 ) The readily stained firmer thread- 

 work, (2) an intermediate clear substance filling 

 up the interstices, ( 3 ) definite and usually globular 

 formations known as nucleoli, ( 4 ) various granules, 

 and (5) a limiting membrane or nuclear wall. 

 These may be briefly touched upon in order. 



( 1 ) The Nuclear Framework (reticulum, trabecu- 

 lar framework, &c.). A mere statement of the 

 different descriptions given of this important part 

 of the nucleus would carry us far beyond the limits 

 of this article. The most marked difference of 

 opinion is this, that some describe the framework 

 as distinctly of the nature of a network, while 

 others are as emphatic in calling it a much-coiled 

 band. A third party unite both views, and re- 

 garding the nucleus as variable, describe a reti- 

 culum at one time and a coiled filament at another. 

 Thus, according to Flemming, Pfitzner, Ret/ins, 

 Leydig, Van Beneden, &c., the nuclear framework 

 is typically a reticulum ; according to Strasburger, 

 Balbiani, and Korschelt, a twisted ribbon is the 

 only or most frequent form ; according to Brass 

 and Rabl, both types may equally occur. A further 

 complication has been emphasised by Zacharias, 

 Pfitzner, Carnoy, and others this, namely, that 

 besides the readily stained threadwork noted above 

 (the so-called chroinatin ). whether this be in the 

 form of a reticulum^fPfitzner) or of a coiled ribbon 

 (Carnoy), there exists another not readily stained 

 framework of achromatin. This had indeed been 

 recognised though Hot insisted on by the first series 

 of investigators. To sum up, it is now generally 

 allowed that the framework or threadwork of the 

 nucleus may exist as a network or as a coil, and 

 that it is in a sense double, consisting of readily 

 stainable chromatin on the one hand, and unstain- 

 able achromatin on the other. It need hardly be 

 added that as there is considerable diversity of 



