o exactly equal halves。 The nuclear membrane then disappears and fibrillar cell…plasma or cytoplasm invades the nuclear area。 In animal cells these fibrillae in the cytoplasm centre on definite bodies （Their existence and their multiplication by fission were demonstrated by E。 van Beneden and Th。 Boveri in 1887。）； which it is customary to speak of as Centrosomes。 Radiating lines in the adjacent cell…plasma suggest that these bodies constitute centres of force。 The cells of the higher plants do not possess such individualised centres； they have probably disappeared in the course of phylogenetic development： in spite of this； however； in the nuclear division…figures the fibrillae of the cell…plasma are seen to radiate from two opposite poles。 In both animal and plant cells a fibrillar bipolar spindle is formed； the fibrillae of which grasp the longitudinally divided chromosomes from two opposite sides and arrange them on the equatorial plane of the spindle as the so… called nuclear or equatorial plate。 Each half…chromosome is connected with one of the spindle poles only and is then drawn towards that pole。 （These important facts； suspected by W。 Flemming in 1882； were demonstrated by E。 Heuser； L。 Guignard； E。 van Beneden； M。 Nussbaum； and C。 Rabl。）
The formation of the daughter…nuclei is then effected。 The changes which the daughter…chromosomes undergo in the process of producing the daughter… nuclei repeat in the reverse order the changes which they went through in the course of their progressive differentiation from the mother…nucleus。 The division of the cell…body is completed midway between the two daughter… nuclei。 In animal cells； which possess no chemically differentiated membrane； separation is effected by simple constriction； while in the case of plant cells provided with a definite wall； the process begins with the formation of a cytoplasmic separating layer。
The phenomena observed in the course of the division of the nucleus show beyond doubt that an exact halving of its substance is of the greatest importance。 （First shown by W。 Roux in 1883。） Compared with the method of division of the nucleus； that of the cytoplasm appears to be very simple。 This led to the conception that the cell…nucleus must be the chief if not the sole carrier of hereditary characters in the organism。 It is for this reason that the detailed investigation of fertilisation phenomena immediately followed researches into the nucleus。 The fundamental discovery of the union of two nuclei in the sexual act was then made （By O。 Hertwig in 1875。） and this afforded a new support for the correct conception of the nuclear functions。 The minute study of the behaviour of the other constituents of sexual cells during fertilisation led to the result； that the nucleus alone is concerned with handing on hereditary characters （This was done by O。 Hertwig and the author of this essay simultaneously in 1884。） from one generation to another。 Especially important； from the point of view of this conclusion； is the study of fertilisation in Angiosperms （Flowering plants）； in these plants the male sexual cells lose their cell…body in the pollen…tube and the nucleus only the sperm…nucleusreaches the egg。 The cytoplasm of the male sexual cell is therefore not necessary to ensure a transference of hereditary characters from parents to offspring。 I lay stress on the case of the Angiosperms because researches recently repeated with the help of the latest methods failed to obtain different results。 As regards the descendants of angiospermous plants； the same laws of heredity hold good as for other sexually differentiated organisms； we may； therefore； extend to the latter what the Angiosperms so clearly teach us。
The next advance in the hitherto rapid progress in our knowledge of nuclear division was delayed； because it was not at once recognised that there are two absolutely different methods of nuclear division。 All such nuclear divisions were united under the head of indirect or mitotic divisions； these were also spoken of as karyo…kineses； and were distinguished from the direct or amitotic divisions which are characterised by a simple constriction of the nuclear body。 So long as the two kinds of indirect nuclear division were not clearly distinguished； their correct interpretation was impossible。 This was accomplished after long and laborious research； which has recently been carried out and with results which should； perhaps； be regarded as provisional。
Soon after the new study of the nucleus began； investigators were struck by the fact that the course of nuclear division in the mother…cells； or more correctly in the grandmother…cells； of spores； pollen…grains； and embryo… sacs of the more highly organised plants and in the spermatozoids and eggs of the higher animals； exhibits similar phenomena； distinct from those which occur in the somatic cells。
In the nuclei of all those cells which we may group together as gonotokonts （At the suggestion of J。P。 Lotsy in 1904。） （i。e。 cells concerned in reproduction） there are fewer chromosomes than in the adjacent body…cells （somatic cells）。 It was noticed also that there is a peculiarity characteristic of the gonotokonts； namely the occurrence of two nuclear divisions rapidly succeeding one another。 It was afterwards recognised that in the first stage of nuclear division in the gonotokonts the chromosomes unite in pairs： it is these chromosome…pairs； and not the two longitudinal halves of single chromosomes； which form the nuclear plate in the equatorial plane of the nuclear spindle。 It has been proposed to call these pairs gemini。 （J。E。S。 Moore and A。L。 Embleton； 〃Proc。 Roy。 Soc。〃 London； Vol。 LXXVII。 page 555； 1906； V。 Gregoire； 1907。） In the course of this division the spindle…fibrillae attach themselves to the gemini； i。e。 to entire chromosomes and direct them to the points where the new daughter… nuclei are formed； that is to those positions towards which the longitudinal halves of the chromosomes travel in ordinary nuclear divisions。 It is clear that in this way the number of chromosomes which the daughter…nuclei contain； as the result of the first stage in division in the gonotokonts； will be reduced by one half； while in ordinary divisions the number of chromosomes always remains the same。 The first stage in the division of the nucleus in the gonotokonts has therefore been termed the reduction division。 （In 1887 W。 Flemming termed this the heterotypic form of nuclear division。） This stage in division determines the conditions for the second division which rapidly ensues。 Each of the paired chromosomes of the mother…nucleus has already； as in an ordinary nuclear division； completed the longitudinal fission； but in this case it is not succeeded by the immediate separation of the longitudinal halves and their allotment to different nuclei。 Each chromosome； therefore； takes its two longitudinal halves into the same daughter…nucleus。 Thus； in each daughter…nucleus the longitudinal halves of the chromosomes are present ready for the next stage in the division； they only require to be arranged in the nuclear plate and then distributed among the granddaughter…nuclei。 This method of division； which takes place with chromosomes already split； and which have only to provide for the distribution of their longitudinal halves to the next nuclear generation； has been called homotypic nuclear division。 （The name was proposed by W。 Flemming in 1887； the nature of this type of division was； however； not explained until later。）
Reduction division and homotypic nuclear division are included together under the term allotypic nuclear division and are distinguished from the ordinary or typical nuclear division。 The name Meiosis （By J。 Bretland Farmer and J。E。S。 Moore in 1905。） has also been proposed for these two allotypic nuclear divisions。 The typical divisions are often spoken of as somatic。
Observers who were actively engaged in this branch of recent histological research soon noticed that the chromosomes of a given organism are differentiated in definite numbers from the nuclear network in the course of division。 This is especially striking in the gonotokonts； but it applies also to the somatic tissues。 In the latter； one usually finds twice as many chromosomes as in the gonotokonts。 Thus the conclusion was gradually reached that the doubling of chromosomes； which necessarily accompanies fertilisation； is maintained in the product of fertilisation； to be again reduced to one half in the gonotokonts at the stage of reduction…division。 This enabled us to form a conception as to the essence of true alternation of generations； in which generations containing single and double chromosomes alternate with one another。
The single…chromosome generation； which I will call the HAPLOID； must have been the primitive generation in all organisms； it might also persist as the only generation。 Every sexual differentiation in organisms； which occurred in the course of phylogenetic development； was followed by fertilisation and therefore by the creation of a diploid or double… chromosome product。 So long as the germination of the product of fertilisation； the zygote； began with a reducing process； a special DIPLOID generation was not represented。 This； however； appeared later as a product of the further evolution of the zygote； and the reduction division was correspondingly postponed。 In animals； as in plants； the diploid generation attained the higher development and gradually assumed the dominant position。 The haploid generation suffered a proportional reduction； until it finally ceased to have an independent existence and became restricted to the role of producing the sexual products within the body of the diploid generation。 Those who do not possess the necessary special knowledge are unable to realise what remains of the first haploid generation in a phanerogamic plant or in a vertebrate animal。 In Angiosperms this is actually represented only by the short developmental stages which extend from the pollen mother…cells to the sperm…nucleus of the pollen…tube； and from the embryo…sac mother…cell to the egg and the endosperm tissue。 The embryo…sac remains enclosed in the diploid ovule； and within th