Hypoblast

Name: The Works of Francis Maitland Balfour, Volume 1 (of 4)
Author: Francis M. Balfour

Definition and stakes

The hypoblast is a crucial germ layer in early embryonic development, forming part of the blastoderm and playing a key role in gastrulation. Francis M. Balfour provided detailed insights into its axial differentiation, explaining how it gives rise to the notochord and interacts with the epiblast and mesoderm. His observations emphasize its layered structure, yolk-associated cells, and dynamic interactions with neighboring tissues.

While Thomas Henry Huxley made significant contributions to embryological theory, Balfour’s precise descriptions of hypoblast structure and function—such as its transformation into the alimentary cavity and its link to the primitive streak—underscore its importance in understanding germ layer organization and vertebrate development.

Quotes about “hypoblast”

Francis M. Balfour, The Works of Francis Maitland Balfour…

“ The hypoblast. Very shortly after the formation of the mesoblastic plates as lateral differentiations of the lower layer cells, an axial differentiation of the hypoblast appears, which gives rise to the notochord very much in the same way as in Amphioxus.
At first the hypoblast along the axial line forms a single layer in contact with the epiblast. Along this line a rod-like thickening of the hypoblast very soon appears (fig. 25, B and C, Ch ́) at the head end of the embryo, and gradually extends backwards. ”

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Source: Gutenberg

Francis M. Balfour, The Works of Francis Maitland Balfour…

“ The hypoblast (hy) has become a thick layer of somewhat cubical cells several rows deep. These cells, especially in front, are characterized by their numerous yolk spherules, and give the impression that part of the area pellucida has been, so to speak, reclaimed from the area opaca. Posteriorly, at the front end of the primitive streak, the thick layer of hypoblast, instead of being continuous with the flattened hypoblast under the primitive streak, falls, in the axial line, into the mesoblast of the primitive streak (Pl. 43, fig. D) . ”

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Source: Gutenberg

Francis M. Balfour, The Works of Francis Maitland Balfour…

“ A transverse section, through about the middle of this blastoderm, is represented in fig. 12 B. The epiblast is seen above. On each side is the section of an atrial tube (at) . Below is the hypoblast which is separated from the yolk especially in the middle line; at each side it is beginning to grow in below, on the surface of the yolk. The space below the hypoblast is the alimentary cavity, the ventral wall of which is formed by the cells growing in at the sides. Between the epiblast and hypoblast are placed scattered mesoblast cells, the origin of which has not been clearly made out. ”

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Source: Gutenberg

Francis M. Balfour, The Works of Francis Maitland Balfour…

“ The continuity of epiblast and hypoblast at the dorsal lip of the blastopore. (2) The continuous conversion of primitive [Pg 284] hypoblast cells into permanent hypoblast, which gradually extends inwards towards the segmentation cavity, and exactly represents the course of the invagination whereby in Amphibia the dorsal wall of the alimentary cavity is formed. (3) The obliteration of the segmentation cavity during the period when the pseudo-invagination is occurring. ”

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Source: Gutenberg

Francis M. Balfour, The Works of Francis Maitland Balfour…

“ The hypoblast can, in most instances, be traced as a distinct layer underneath the primitive streak, although it is usually less easy to follow it in that region than elsewhere, and in some cases it can hardly be distinctly separated from the superjacent cells. ”

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Source: Gutenberg

Francis M. Balfour, The Works of Francis Maitland Balfour…

“ The notochord does not long remain attached to the hypoblast, and the separation between the two is already effected for the greater part of the length of the embryo by the stage represented in fig. 129. Fig. 126 represents a series of sections through this embryo.
Fig. 127. Diagrammatic longitudinal section of an embryo of Lacerta.
pp. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypoblast; ep. epiblast of the medullary plate; pr. primitive streak. In the primitive streak all the layers are partially fused. ”

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Source: Gutenberg

Francis M. Balfour, The Works of Francis Maitland Balfour…

“ The presence of food-yolk is much more frequent in the hypoblast than in the epiblast; and it is well known that a large number of the changes in early development are caused by food-yolk. If, therefore, the mesoblast has been derived from the hypoblast, many more changes might be expected to have been introduced into its early development than if it had been derived from the epiblast. ”

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Source: Gutenberg

Francis M. Balfour, The Works of Francis Maitland Balfour…

“ When the segmentation results in the formation of a blastosphere, one half of the blastosphere may be pushed in towards the opposite half, and a gastrula be thus produced (fig. 199, A and B) . This process is known as embolic invagination. Another process, known as epibolic invagination, consists in epiblast cells growing round and enclosing [Pg 334] the hypoblast (fig. 200) . This process replaces the former process when the hypoblast cells are so bulky from being distended by food-yolk that their invagination is mechanically impossible. ”

Hypoblast

Definition and stakes

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