Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    and the yolk as a mass of enveloping substance. A unanimity of views in this matter was brought about only after the general conception of cell had received in Histology a more precise definition. This was due especially to more accurate knowledge of the processes of cell-formation gained through the works of Nagkli, Kolliker, Eemak, Leydig, and others. The interpretation of eggs witli separate formative and nutritive yolk, and with partial cleavage, occasioned especial difficulty. Two antagonistic views in this matter have existed for a long time. According to one view, eggs with polar nutritive yolk (the eggs of Eeptiles, Birds, etc.) are compound structures, which cannot be designated as simple cells. Only the formative yolk, together with the germinative vesicle, is comparable with the Mammalian egg; the nutritive yolk, on the contrary, is something new, superposed upon the cell from without, a product of the follicular epithelium. The spherules of the white yolk are explained as uninuclear and multinuclear yolk-cells. The formative and nutritive yolk together are comparable with the entire contents of the Geaafian vesicle of Mammals. H. Meckel, Allen Thomson, ECKER, Steicker, His, and others, have expressed themselves in favour of this view with slight modifications in the details. According to the opposite view of Leuckart, Kolliker, Gegenbaur, Haeckel, van Beneden, Balfour, and others, the Bird's egg is just as truly a simple cell as the egg of a Mammal, and the comparison with a Graafian follicle is to be rejected. The yolk never contains enclosed cells, but only nutritive components. As Kolliker, especially in opposition to His, has shown, the white-yolk spherules contain no structures comparable with genuine cell-nuclei; and therefore cannot be interpreted as cells. As Gegenbaur already in 1861 sharply formulated it: The eggs of Vertebrates with partial cleavage are on that account essentially no more compound structures than those of the remaining Vertebrates; they are nothing else than enormous cells peculiarly modified for special purposes, but which never surrender this their real character. There would be no change in this interpretation, even if it should prove to be that the yolk was formed in part from the follicular epithelium, and was set free from the latter as a sort of secretion. In that event we should have to do with a special method of nutrition of the egg, the cell-nature of which cannot on that account be called in question. Various components of the yolk have received special names. Eeiciiert first distinguished as formative yolk the finely granular mass, which, in the Bird's egg, contains the germinative vesicle, and forms the germ-disc, because it alone undergoes the process of cleavage, and produces the embryo. The other chief mass of the egg he called nutritive yolk, because it does not break up into cells, and because subsequently, enclosed in a yolk-sac, it is consumed as nutritive material. Afterwards His introduced for these the names chief germ and accessory germ {Iiau])t- tmd Neheiiltcini). Whereas the nomenclature of IIeichert and His is applicable only to eggs with polar arrangement of nutritive yolk, VAN Benbden (1870) has undertaken the division of the substance of the egg from a more general standpoint. He distinguishes between the protoplasmic matrix of the egg, in which, as in every cell in general, the vital processes take place, and the reserve and nutritive materials, which are stored up in the protoplasm in the form of granules, plates, and balls, and which he designates as deutoplasm. Every egg possesses both components, only in diHerent proportions, in varied forms and distribution. Balfoue has selected this latter condition as a basis for
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