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origin of heterotype chromosomes. The differences between the two schools of thought (" telosynaptists " and " parasynaptists ") rest on the interpretation of the heterotype prophases (Farmer 1912) (fig. 4). Telosynaptists regard the paired threads of the

Anaphase Telophase Rest Prophase Metaphase Anaphase

FIG. i.

-Diagram of a Somatic Division

presynaptic and synaptic phases (fig. 2) as the associating halves (homologous with the threads of the somatic prophases) of a somatic chromosome which separated during the preceding telophase, and maintain that the conjunction of the two entire homologous somatic chromosomes takes place prior to, and during second contraction (fig. 3). Consequently the associating threads of synapsis only separate as daughter chromosomes on the homotype (2nd meiotic) spindle.

FIG. 2.

FIG. 2. Synapsis (Osmunda regalis)

(After Gregoire) (From La. Cellule, vol. XXIV.)

FIG. 3. FIG. 3. Second Contraction

(Smilacina)

(After Lawson)

(From Trans. Roy. Soc. Edin- burgh, vol. XLVIII.)

" Parasynaptists " (Gregoire's school), on the other hand, regard the parallel threads of the presynaptic and synaptic prophases (fig. 2) as the pairing of two entire homologous somatic chromosomes which will separate on the heterotype (ist meiotic) spindle. They attach no significance to the second contraction phase. (Stomps 1911, Davis 1911, Lawson 1912, Gregoire 1912, Fraser 1914, Noth- nagel 1916, Digby 1919.)

Anaphas* Tetophe Prophase SympMB

(Ust Premiottc (Last Premiotic (Heterotype) Division) Division)

.^

Hollow Second Htterotype Himotyp* Spindl. Spirm* Contraction Spindle

'Parasynaptic' Interpretation

FIG. 4. Diagram of Meiotic Phase.

The doctrine initiated by Boveri concerning the individuality of chromosomes is now widely supported. It is based on three main facts: (a) the continuity of chromosomes can sometimes be traced from telophase into the ensuing prophase, especially in rapidly dividing tissues; (6) the specific number of chromo- somes, in any given animal or plant, recurs at each mitosis with extraordinary regularity. 1

Tischler (1915) and Ishikawa (1916) have collated the numbers recorded in plants : (c) chromosomes with striking individual charac- ters of size and shape, distinguishing them from others in the nucleus,

occur in many animals and plants. These characteristic chromo- somes invariably reappear at every mitosis.

Recently much attention has been given to the significance of specific numbers of chromosomes. It has been conclusively shown that closely allied species of many families have related chromosome numbers, i.e. multiples of a common factor. Thus diploid, triploid, tetraploid, 2 etc., forms are distinguished (Marchal 1912, Gregory 1914, Winge 1917, Holmgren 1919, Kuwada 1919, Rosenberg 1920). This multiplication of chromosomes may prove to have an important bearing on mutation, e.g. Primula sinensis 12 (haploid number), P. sinensis (giant) 24 (haploid number), Crepis virens 3 (haploid number), Crepis tectorum 4 (haploid number), Crepis rubra 5 (haploid number), Oenothera Lamarckiana 7 (haploid number), O. gigas 14 (haploid number) (fig. 5).

1 The number of chromosomes is inconstant in degenerating tissues such as endosperm and tapetum ; nuclei with a double number may

FIG. 5.

Oenothera Lamarckiana Oenoikera gigas

(7 pairs of chromosomes) (14 pairs of chromosomes)

(Interkinesis between Heterotype and Homotype Mitoses) (After Davis) (From Ann. of Bo/., vol. XXV.)

As regards dimensions, the width of chromosomes, both in ani- mals and plants, is inconstant and more or less variable, and is in no way correlated with phylogenetic affinity (Farmer and Digby 1914). Among other points of importance, recent work on hybrids (Rosenberg 1917) confirms previous observations, that the offspring

of parents with an unlike number of chromosomes show irregular meiotic divisions. The classical experi- ments of Nemec who, by submit- ting root tips to the action of chloral hydrate, produced multi- nucleate cells and abnormal mitoses, have been extended (Sakamura 1920). Extrusion of particles of nuclear substance, especially as glob- ules from the nucleolus, has been repeatedly observed in animal and plant cells (fig. 6) ; this phenomenon probably denotes some important physiological interchange between nucleus and cytoplasm (von Der- schau 1915 and 1920).

Many authorities believe that chromosomes determine the segre- gation of Mendelian characters and that mitosis provides precisely the mechanism required. Gates and

FIG. 6. Extrusion of Nu- clear Substance (Funaria hygrometrica) (After von Derschau)

(From Archil) fur Zellfor- Thomas (1914) have shown that schung, vol. XIV.) those Oenothera mutants lata and

semilata, which possess an extra

chromosome, i.e. 15 instead of 14 (somatic number), always have a characteristic type of foliage and habit (fig. 7).

Bibliography. Two new text-books have been published (Agar 1920, and Doncaster 1920), and Wilson's Cell has been reprinted (1919). On special points, the following may be mentioned: Davis, " Cytological Studies on Oenothera III. A Comparison of the Reduction Divisions of Oenothera Lamarckiana and O. gigas," Ann. of Bot. (1911, vol. XXV.) ; von Derschau, " Der Austritt ungeloster Substanz aus dem Zellkerne (Eine zusammenfassende Studie)," Archiv f. Zellforschung (1915, vol. XIV.), " Pflanzliche Plasmastruk- turen und ihre Beziehungen zum Zellkern," Flora (1920. vol. XIII.); Digby, " On the Archesporial and Meiotic Mitoses of Osmunda," Ann. of Bot. (1919, vol. XXXIII.); Farmer, " Telosynapsis and Parasynapsis," Ann. of Bot. (1912, vol. XXVI.); Farmer and Digby, " On Dimensions of Chromosomes considered in relation to Phylog- eny," Phil. Trans. Series B. (1914, vol. CCV.) ; Fraser, "The Be- haviour of the Chromatin in the Meiotic Divisions of Vicia Faba," Ann. of Bot. (1914, vol. XXVIII.); Fraser and Snell, " The Vegeta- tive Divisions in Vicia Faba." Ann. of Bot. (1911, vol. XXV.); Gates and Thomas, " A Cytological Study of Oenothera mut. lata and Oe. mut. semilata in Relation to Mutation," Quart. Journ. Micr. Sci. (1914, vol. LIX.); Gregoire, " Les Phenomenes de la metaphase et de 1'anaphase dans la caryocinese somatique," Ann. Soc. Scientif. de Bruxelles (1912, vol XXXVI.), "La verite du

occur in root tips (Strasburger 1911).

2 Winkler (1920) has produced tetraploid forms artificially.