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The cell in development and inheritance . spindle-hbres, to be largely composed. In support of this may bementioned, besides the mode of formation of the partition-walls in the division of plant-cells, Harpers (97) very interesting observations on the formation of the ascospores inErysiphe (Fig. 33), where the spore-membrane appears to arise directly from the astralrays. 56 GENERAL SKETCH OF THE CELL focussed at the centrosome. On this basis he endeavours to explainthe position and movements of the nucleus, the succession of division-planes, and many related phenomena.^ Hatschek (88) and Rabl

The cell in development and inheritance . spindle-hbres, to be largely composed. In support of this may bementioned, besides the mode of formation of the partition-walls in the division of plant-cells, Harpers (97) very interesting observations on the formation of the ascospores inErysiphe (Fig. 33), where the spore-membrane appears to arise directly from the astralrays. 56 GENERAL SKETCH OF THE CELL focussed at the centrosome. On this basis he endeavours to explainthe position and movements of the nucleus, the succession of division-planes, and many related phenomena.^ Hatschek (88) and Rabl Stock Photo

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The cell in development and inheritance . spindle-hbres, to be largely composed. In support of this may bementioned, besides the mode of formation of the partition-walls in the division of plant-cells, Harpers (97) very interesting observations on the formation of the ascospores inErysiphe (Fig. 33), where the spore-membrane appears to arise directly from the astralrays. 56 GENERAL SKETCH OF THE CELL focussed at the centrosome. On this basis he endeavours to explainthe position and movements of the nucleus, the succession of division-planes, and many related phenomena.^ Hatschek (88) and Rabl (89, 92), on the other hand, have ad-vanced a quite different hypothesis based on physiological considera-tions. By cell-polarity these authors mean, not a predeterminedmorphological arrangement of parts in the cell, but a polar differen-tiation of the cell-substance arising secondarily through adaptation ofthe cell to its environment in the tissues, and having no- necessaryrelation to the polarity of Van Beneden (Fig. 22, B, C). This is.

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RM2AJGTJP–The cell in development and inheritance . there is nobreach in the continuity of the chromatin from one cell-generation toanother. With the achromatic figure the case is widely different.The material of the spindle and asters must be derived from thenucleus, from the cytoplasm, or from both; and most of the earlierresearch was devoted to an endeavour to decide between thesepossibilities. The earliest observers (73-75) supposed the achro-matic figure to disappear entirely at the close of cell-division, andmost of them (Biitschli, Strasburger, Van Beneden, 75) believedit to be re-formed at each

RM2AJF7BR–The cell in development and inheritance . age-nucleus of the ovum, then gives rise by division to both macronucleiand micronuclei of the offspring of the conjugating animals (Fig. 109). These facts may be illustrated by the conjugation of ParamoEciiimcmidatum, which possesses a single macronucleus and micronucleus,and in which conjugation is temporary and fertilization mutual. Thetwo animals become united by their ventral sides and the macronu-cleus of each begins to degenerate, while the micronucleus dividestwice to form four spindle-shaped bodies (Fig. no. A, B). Three ofthese degenerate, fo

RM2AJFJTH–The cell in development and inheritance . (97) gives a somewhat different account of the process, believing that thering is derived from the mid-body of the last mitosis. Meves and McGregor have, however,shown that the ring and mid-body coexist in the early spermatids (Fig. 84), which seemsdecisive against Hermanns conclusion. ^ Hermann finds also the ring observed in the salamander, and believes it to be the mid-body. The middle-piece is regarded by him as a product of the spindle-remains, but onboth these points he is contradicted by Suzuki. 168 THE GERM-CELLS archoplasm of the spermatid. Su

RM2AJE35K–The cell in development and inheritance . t harmonizesfurther with Driiners hypothesis of the active elongation of thespindle in mitosis (p. 105). There are, however, a large number offacts which show that neither the form of the protoplasmic mass nor ^ Cf. Francotte on the polar bodies of Turbellaria, p. 235,2 For a good review and critique, see Jennings, 97, 376 CELL-DIVISION AND DEVELOPMENT the distribution of metaplasmic materials is sufficient to explain theposition of the spindle, whether with reference to the direction or theinequality of the cleavage. As regards the direction of the sp

RM2AJGAKH–The cell in development and inheritance . farreached, whether by observation or by experiment, are more than thefirst naive attempts to analyze a group of most complex phenomenaof which we have little real understanding. 1 See pp. 306, 308. 2 cf. p. 215. ^Cf. p. 213. 112 CELL-DIVISION 2. Division of the Chromosomes In developing his theory of fibrillar contractility, Van Benedenexpressed the view — only, however, as a possibility — that thesplitting of the chromosomes might be passively caused by the con-tractions of the two sets of opposing spindle-fibres to which each isattached.^ Later obse

RM2AJEFDP–The cell in development and inheritance . in the configurationof the network, and he described the nucleus as showing a distinctpolarity having a pole corresponding with the point toward whichthe apices of the chromosomes converge {i.e. toward the centrosome),and an antipole (Gegenpol) at the opposite point {i.e. toward theequator of the spindle) (Fig. 22). Rabls hypothesis was preciselyformulated and ardently advocated by Boveri in 1887 and 1888, andagain in 1891, on the ground of his own studies and those of VanBeneden on the early stages of Ascaris. The hypothesis was supported MORPHOLOGICA

RM2AJGW0P–The cell in development and inheritance . Fig. 26.— Diagrams of the later phases of mitosis. G, Metaphase; splitting of the chromosomes (c./.). n. The cast-off nucleolus. H. Ana-phase ; the daughter-chromosomes diverging, between them the interzonal-fibres (/./.), or centralspindle ; centrosomes already doubled in anticipation of the ensuing division. /. Late anaphaseor telophase, showing division of the cell-body, mid-body at the equator of the spindle and begin-ning reconstruction of the daughter-nuclei. J, Division completed. ally known as arcJioplasm (Boveri, 88), but this term is not appl

RM2AJGFRH–The cell in development and inheritance . emann, and Galeotti have especially pointed out,that they are of frequent occurrence in abnormal growths such ascancers and tumours. The abnormal forms of mitoses are arranged by Hansemann in twogeneral groups, as follows: (i) asymmetrical mitoses, in which thechromosomes are unequally distributed to the daughter-cells, and (2)multipolar mitoses, in which the number of centrosomes is more than 98 CELL-DIVISION two, and more than one spindle is formed. Under the first group areincluded not only the cases of unequal distribution of the daughter-chromosom

RM2AJEMWA–The cell in development and inheritance . r may bereconciled with his own. The essence of Strasburgers interpretation is as follows. In theprophases of the first division the chromosomes first undergo a longitudinal division, shortento form double rods, and then again split lengthwise in a plane at right angles to the first.The following stages vary even in the same species {Lilium); and here lies the explanationof much of the divergence between the accounts of different observers. (0 Ir* the typicalcase, the chromosomes are placed radially, with one end next the spindle; and, during themetaph

RM2AJGNPF–The cell in development and inheritance . fibres, each of which terminates in a deeply staining granule. Nearlysimilar spindles have been described by Strasburger (80) in Spirogyra, and in the embryo-sac of Monotropa. It is not impossible that such spindles may represent a type intermediatebetween the cormoptyte and thallophyte types of Mottier. 1 Cf. p. 306. ^ I have had the privilege of examining some of Meads beautiful preparations. DETAILS OF MITOSIS 85 with the facts described in plants, it indicates that the forces involvedin spindle-formation are far more complex than Van Benedens andBo

RM2AJH8EY–The cell in development and inheritance . revealedthe existence within the nerve-cell of networks far more complex and definite than wasformerly supposed, and showing definite relations to incoming and outgoing fibrillae withinthe substance of the nerve-fibres. THE CYTOPLASM 49 afforded by the appearance of the spindle and asters in cross-section.In the early stages of the o.^^ of Nereis, for example, the astral raysare coarse anastomosing fibres that stain intensely and are thereforevery favourable for observation (Fig. 60). That they are actual fibresis, I think, proved by sagittal sections

RM2AJGJDK–The cell in development and inheritance . he micro-nuclei of Parantceciuni, which at first contain a large nucleolus and afterward a conspicuous pole-plate at either end of the spindle(Fig. 38, D-H). The origin of the pole-plates was not, however,positively determined. A corresponding dividing body is found inCcratijini (Lauterborn, 95), and as in the Infusoria the entirenucleus transforms itself into a fibrillar spindle-like body. 92 CELL-DIVISION Still simpler conditions are found in some of the flagellates.^ InChilonwnas the sphere may still be regarded as intranuclear, since itlies in the