Archive image from page 191 of Cytology, with special reference to. Cytology, with special reference to the metazoan nucleus cytologywithspec00agar_0 Year: 1920 176 CYTOLOGY CHAP. intimate nature of the syndesis proper, has obviously the function of bringing the homologous chromosomes in pairs on to the meiotic spindle. This being effectively achieved by this late prophase pairing, we are compelled to look for another function for syndesis in these cases, and this function, as indicated above, we believe to be the exchange of hereditary factors. In what way may we conceive that this hypot
Archive image from page 191 of Cytology, with special reference to. Cytology, with special reference to the metazoan nucleus cytologywithspec00agar_0 Year: 1920 176 CYTOLOGY CHAP. intimate nature of the syndesis proper, has obviously the function of bringing the homologous chromosomes in pairs on to the meiotic spindle. This being effectively achieved by this late prophase pairing, we are compelled to look for another function for syndesis in these cases, and this function, as indicated above, we believe to be the exchange of hereditary factors. In what way may we conceive that this hypothetical interchange of factors is effected ? Two possibilities have been suggested—the first is that which naturally presents itself, namely, that while the chromo- somes are longitudinally apposed to one another in parasyndesis, an exchange of chromatin units takes place analogous to the exchange of nuclei between two conjugating Infusoria. The other suggestion was first made by Janssens (1909) (chiasma- typie) on purely cytological grounds. Since the homologous chromosomes separating after syndesis are often spirally twisted round one another (strepsitene stage), he suggested that fusion may occur at the points of junction, and 'that when these break apart at final separation the chromo- somes may be composed of alternate segments of the original chromosomes (Fig. 78). A view similar to Janssens' has been supported by Morgan, etc., as forming a cytological foundation for certain breeding results with Droso- phila and other forms. It is known as the ' crossing over ' hypothesis, and is supported by the following experimental evidence. A number of cases are now known in which characters which can be separated in crossing, nevertheless show a preference to remain in the combinations in which they were present in the parents, rather than to become rearranged in different combinations. Thus if two forms, differing as regards two characters, are crossed, say4B x ab, the F1 hybrid
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