By Alfred Kühn
"1 Jind released lectures insipid. " ALEXANDER VON HUMBOLDT ta LOUIS AGASSIZ, January 15, 1840 many stuff communicate opposed to the book of lectures. The influence of the spoken be aware is especially diverse from that of the written note. Repeated recapitulations, which aid the listener less than stand, are pointless, and the overtones of speech disappear in print. A lecture, quite whilst no longer intended for newcomers and never provided in coaching for a try, allows the privilege of subjective selection, and while released it really is rightly open to feedback. So it has no longer been effortless for me to figure out to put up those lectures, which i've got given in a number of kinds in Gattingen after which back in Tiibingen. in any case, the determining elements have been the urging of my scholars, the encouragement of peers, lots of whom i used to be extremely joyful to determine between my listeners, and the want of my writer, with whom i've got loved a pleasant courting for a very long time. the ultimate kind of the textual content is largely a duplicate, and hence reallectures are the guts of it. If the lectures had now not been taken down again and again, this publication do not have seemed; 1 couldn't have undertaken a textbook-sized compilation of this fabric. With all of the alterations, the association, the road of inspiration, and of ten the composition of the lectures themselves have been retained. they could bear in mind to my listeners of outdated the hours we spent jointly.
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In Datura stramonium a recessive allele causes the complete absence of pairing in the pollen mother cells and in the macrospore mother cells. The unpaired chromosomes are distributed irregularly in the first division; in the second division a normal segregation of the chromatids takes place. The number of chiasm ata within the normal range is also genetically determined; it varies under constant growing conditions among different lines of Viciafaba. 45b). If the dominant allele (Co) for small flowers in Oenothera suaveolens is crossed into Oenothera hookeri, the tendency for pairing is reduced and the percentage of asynapsis at diakinesis rises (Table 2).
Sin ce pairing takes place only in spore-forming f anaphase. (After KUHN, 1965) cells, gamete-forming cells, and zygotes. The nature -+ of this state is unknown; in unicellular organisms it -+ -+ + -+ -+ -+ can be induced experimentally, as we shall see. -+ The further course of meiosis, especially the mutual coiling of the chromosomes and chiasma formation, can be influenced by various conditions. Certain genes are also involved: in an asynaptic strain of maize 91 the homologous chromosomes begin to pair -+ -+ -+ -+ -+ normally but separate again in pachytene without -+ -+ -+ forming chiasmata.
Evidently the fibrils belonging to one chromonema come together or remain together as the tangle is replaced by a more obvious order. The orienting fibrils are about a hundred A thick in all sperms examined, and they of ten turn out to be made up of two smaller fibrils about forty A thick, which are closely intertwined (Fig. 31 c). Even in functional nuclei containing a good deal of nuclear sap, as in Amoeba proteus, helically coiled fibrils of similar dimensions have been found in thin sections in the e1ectron microscope (Fig.