. Text-book of embryology. Embryology. 246 INVEETEBEATA chap. 96. per cent alcoholâa treatment which resulted in the embryonic area being coloured dark blue, while the rest of the egg was nearly- colourless. Such eggs were then used for sections. When it was desired to have whole mounts of the embryonic area, the chorion was carefully removed from the individual egg by means of a fine needle, the eggs were then stained for twenty-four hours in borax carmine, and differentiated for the same period in acid alcohol. For cutting sections the fragments of the cocoon, in which all the contained eggs
. Text-book of embryology. Embryology. 246 INVEETEBEATA chap. 96. per cent alcoholâa treatment which resulted in the embryonic area being coloured dark blue, while the rest of the egg was nearly- colourless. Such eggs were then used for sections. When it was desired to have whole mounts of the embryonic area, the chorion was carefully removed from the individual egg by means of a fine needle, the eggs were then stained for twenty-four hours in borax carmine, and differentiated for the same period in acid alcohol. For cutting sections the fragments of the cocoon, in which all the contained eggs were parallel to one another, with the future head ends pointing in the same direction, were passed through xylol into paraffin. Wheeler mentions that when he used paraffin melting at 55°, the yolky contents of the egg took on a gummy consistency which rendered it specially suitable for cutting, and that he got perfect sections. This must be regarded as a somewhat exceptional circumstance, because yolk is usually apt to become very brittle on heating, and to break up into small fragments under the stroke of the knife, hence, usually, in dealing with yolky eggs, preliminary embedding in celloidin, as described in Chap. II., is desirable. The egg of Donacia, like that of most insects, is of an elongated oval form, and the nucleus is situated near the centre, surrounded by an island of cytoplasm. When the egg is fertilized the zygote nucleus begins to divide and gives rise to many nuclei, each sur- rounded by its cytoplasmic island. In Doryphora, Wheeler got every stage from the first and was able Fig. 193.âPortion of a sagittal section through the tO observe that the divisions developing egg of Doryphora {Leptinotarsa) of the daughter nUclei are decemlineata before the formation of the blasto- â4. fl_âi ;irminna derm. (After Wheeler.) ^^ ^^^^^ StriCtlysynChrOnOUS, , ^ ,, , , , SO that m a given egg all ci/t, peripheral layer of cytoplasm; K, nuclei m
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