The evolution of man : a popular scientific study / by Ernst Haeckel. Translated from the 5th (enl.) ed. by Joseph McCabe.

  • Haeckel, Ernst, 1834-1919.
Date:
1905
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Licence: In copyright

Credit: The evolution of man : a popular scientific study / by Ernst Haeckel. Translated from the 5th (enl.) ed. by Joseph McCabe. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

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    enters on Us second stage (the metovum), which is many times larger than the first, but still only a single enlarged cell. Through the accumulation of the store of yellow yelk within the ball of protoplasm the nucleus it contains (the germinal vesicle) is forced to the surface of the b^ill. Here it is surrounded by a small quantity of protoplasm, and with this forms the lens-shaped formative yelk (Fig. 15 V). This is seen on the yellow yelk-ball, at a certain point of the surface, as a small round white spot—the tread (cicatricula). From this point a thread- like column of white nutritive yelk (d), which contains no yellow yelk-granules, and is softer than the yellow food-yelk, proceeds to the middle of .the yellow yelk- ball, and forms there a small central globule of white yelk (Fig. 15 rf}.- The whole of this white yelk is not sharply separated from the yellow yelk, which (Fii 57). First two equal segmentation- cells (A) are formed from the ovum. These divide into four (B), then into eight, sixteen (C), thirty-two, sixty-four, and so on. The cleavage of the cells is always preceded by a division of their nuclei. The cleavage surfaces between the segmentation-cells appear at the free surface of the tread as clefts. The first two divisions are vertical to each other, in the form of a cross (B). Then there are two more divisions, which cut the former at an angle of forty-five degrees. The tread, which thus becomes the germinal disk, now has the appearance of an eight-rayed star. A circular cleavage next taking place round the middle, the eight triangular cells divide into sixteen, of which eight are in the middle and eight distributed around (C). -Afterwards circular clefts and radial clefts, directed towards the centre, alternate more or less shows a slight trace of concentric layers in the hard-boiled egg (Fig. 15 c). We also find in the hen's egg, when we break the shell and take out the yelk, a round small white disk at its surface which corresponds to the tread. But this small white germinal disk is now further developed, and is really the gastrula of the chick. The body of the chick is formed from it alone. The whole white and yellow yelk-mass is without any significance for the formation of the embryo, it being merely used as food by the developing chick. The clear, glarous mass of albumin that surrounds the yellow yelk of the bird's egg, and also the hard chalky shell, are only formed within the oviduct round the impregnated ovum. When the fertilisation of the bird's ovum has taken place within the mother's body, we find in the lens-shaped stem-cell the progress of flat, discoid segmentation irregularly (D, E). In most of the amniotes the formation of concentric and radial clefts is irregular from the very first ; and so also in the hen's egg. But the final outcome of the cleavage-process is once more the formation of a large number of small cells of a similar nature. As in the case of the fish-ovum, these segmentation-cells form a round, lens- shaped disk, which corresponds to the morula, and is embedded in a small depression of the white yelk. Between the lens-shaped disk of the morula-cells and the underlying white yelk a small Cfivity IS now formed by the accumulation ot fluid, as in the fishes. Thus we get the peculiar ruid not easily recognisable blastula of the bird (Fig. 58). The small segmentation-cavity (fh) is very flat and much compressed, the upper or dorsal \vall ((hv) IS lormed of a single layer of clear, distinctly separated cells; this