The Evolution of Man — Volume 1 by Ernst Heinrich Philipp August Haeckel
page 180 of 358 (50%)
page 180 of 358 (50%)
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5. Although the mass of the food-yelk may be very large in the ova of
the discoblastic vertebrates, nevertheless in every case a blastula is developed from the morula, as in the holoblastic ova. 6. Also, in every case, the gastrula develops from the blastula by curving or invagination. 7. The cavity which is produced in the foetus by this curving is, in each case, the primitive gut (progaster), and its opening the primitive mouth (prostoma). 8. The food-yelk, whether large or small, is always stored in the ventral wall of the primitive gut; the cells (called "merocytes") which may be formed in it subsequently (by "after-segmentation") also belong to the inner germinal layer, like the cells which immediately enclose the primitive gut-cavity. 9. The primitive mouth, which at first lies below at the lower pole of the vertical axis, is forced, by the growth of the yelk, backwards and then upwards, towards the dorsal side of the embryo; the vertical axis of the primitive gut is thus gradually converted into horizontal. 10. The primitive mouth is closed sooner or later in all the vertebrates, and does not evolve into the permanent mouth-aperture; it rather corresponds to the "properistoma," or region of the anus. From this important point the formation of the middle germinal layer proceeds, between the two primary layers. The wide comparative studies of the scientists I have named have further shown that in the case of the discoblastic higher vertebrates |
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