The blastosphere of the frog is like what the blastosphere of amphioxus would be, if the future hypoblast cells were enormously larger through their protoplasm being diluted with yolk.
The hypoblast is the lining of the intestine and of the glands which open into it; and the material of the notochord is also regarded, as hypoblast.
In all the higher animals a layer of cells makes its appearance between the hypoblast and the epiblast, and is termed the mesoblast.
The first or hyomandibular pouch, placed between the mandibular and hyoid arches, has rather the character of a double layer of hypoblast than of a true pouch, though in parts a slight space is developed between its two walls.
As to the origin of the notochord, our main result is that this structure is formed as an actual thickening of the primitive hypoblast of the anterior part of the area pellucida.
We called attention to a change in the character of the hypoblast cells of this region as taking place at the end of the last stage.
At the hind end of the shield a somewhat triangular primitive streak is formed by the fusion of the epiblast and hypoblast with a number of cells between them, which are probably derived from the lower rows of the segmentation cells.
This structure is in main derived from a proliferation of epiblast cells, but Koller admits that some of the cells just above the hypoblast in the region of the Sichelknopf are probably derived from the hypoblast.
This layer becomes in the sections following still more definite, and forms part of the continuous layer of hypoblast present in the region of the primitive streak.
It might have been anticipated that all the layers would fuse along the line of the primitive streak, and that the hypoblast as well as part of the mesoblast would grow out from it.
The growth outwards of the mesoblast from the axis of the primitive streak is probably a remnant of the invagination of the hypoblast and mesoblast from the lip of the blastopore in Amphibia, &c.
Near the front border of the area pellucida (1) it has the general characters of the hypoblast of the duck's blastoderm just described.
My theory, then, makes the impossible assertion that what was hypoblast in the arthropod has become epiblast in the vertebrate, and what was epiblast in the arthropod has become hypoblast in the vertebrate.
If by hypoblast we only mean the internal surface or alimentary canal and its glands, etc.
What is the morphological criterion by which hypoblast can be distinguished from epiblast, or mesoblast from either?
These layers are strictly morphological conceptions, and are stated to be homologous in all cases, so that the hypoblast of one animal must be homologous to the hypoblast of another.
At the edge of the area pellucida the hypoblast becomes continuous with a thickened rim of material, underlying the epiblast, and derived from the original thickened edge of the blastoderm and the subjacent yolk.
Amongst the more important of these are the following:--(1) The continuity of epiblast and hypoblast at the dorsal lip of the blastopore.
In all cases the notochord and the hypoblast below it unite with the front end of the primitive streak; with which also the two lateral plates of mesoblast become continuous.
At its close the epiblast cells, as they may now be called, are clear, and have an irregularly cubical form; while the hypoblast cells are polygonal and granular, and somewhat larger than the epiblast cells.
The hypoblast in the region of the ridge is formed of two layers of cells, the ridge being entirely due to the uppermost of the two.
There are no important points of difference in the relations of the hypoblast between the Anura and Urodela.
The appearance of part of the mesoblast before the formation of the primitive streak is a process of the same nature as the differentiation of hypoblast and mesoblast in Elasmobranchii without an invagination.
Up to about the time when the enclosure of the hypoblast by the epiblast is completed, no external traces are visible of any of the organs of the embryo; but about this time, i.
In the later stages (fourth day and onwards) the whole germinal wall is stated to break up into columnar hypoblast cells, each of them mainly formed of one of the vesicles just spoken of.
The irregular hypoblast spheres of the unincubated blastoderm flatten themselves out, and unite into a definite hypoblastic membrane (fig.
Continued back from this point is a streak of fused mesoblast and epiblast to the under side of which a linear thin layer of hypoblast is more or less definitely attached.
Where this prominence becomes first apparent the epiblast and hypoblast are united together.
After the separation of those parts the remainder of the hypoblast forms the wall of the mesenteron.
Older blastoderm with embryo in which hypoblast and mesoblast are distinctly formed, and in which the alimentary slit has appeared.
The notochord (ch) is seen as an independent cord for the greater part of the length of the embryo, but falls into the hypoblast shortly in front of the neurenteric passage.
At this pole a solid involution of epiblast next becomes formed, to which a layer of hypoblast becomes applied.
In delamination, when the segmentation is not uniform, or when a solid morula is formed, the differentiation of the epiblast and hypoblast is effected by the separation of the central solid mass of cells from the peripheral cells (fig.
In Criodrilus thehypoblast has at first no lumen, but this becomes very soon established.
At the sides of the hypoblast the mesoblast has become established, probably as two lateral bands.
The hypoblast and epiblast cells are distinguished very early, but soon become of nearly the same size.
The mesenteries consist of a fold of hypoblast with a prolongation of the epiblast between the two limbs of the fold.
Of the three layers the epiblast and hypoblast are to be regarded as the primary.
The splanchnic mesoblast follows the hypoblast in its growth, and gives rise to the connective tissue and muscular parts of the walls of the alimentary tract.
According as there is much or little food-yolk the hypoblast spheres are either very bulky or the reverse.
When the segmentation is regular, and results in the formation of a blastosphere, the epiblast and hypoblast are usually differentiated from the uniform cells forming the wall of the blastosphere in one of the two following ways.
As the epiblast spreads over the hypoblast the mesoblastic sheet gives way in the middle, and the mesoblast remains as a ridge of cells at the edge of the epiblastic cup.
The central cell itself gives origin to the hypoblast cell of the adult, and the peripheral cells to the epiblast.
Adjoining this thickened layer of epiblast a special thin layer of hypoblast becomes differentiated, which in opposition to the main mass of large nutritive cells forms the true hypoblastic epithelium (fig.
The bulkiness of the hypoblast cells necessitates a modification of the normal process of embolic invagination, and causes another process to be substituted for it, viz.
During the above changes a fifth hypoblast cell is formed by the division of one of those already present; and at a later period four of the hypoblast cells give rise within the nearly closed blastoporic area to four small cells.
The origin of the hypoblast in Insects has not as yet been clearly determined.
The hourglass-shaped rudiments of the hypoblast become pushed upwards by those foldings-in of the epiblast which form towards the anterior and posterior ends of the embryo, and give rise to the stomodæum and proctodæum.
These rudiments have their convex ends directed away from each other, while their edges are approximated and gradually meet so as to form a continuous hypoblast beneath the mesoblast.
The muscles and skeleton and the reproductive cells arise from a set of cells known as the mesoblast, that are formed chiefly from the hypoblast, and that push their way in between the hypoblast and epiblast.
Portion of the hypoblast of a thirteen hours' blastoderm, treated with silver nitrate, shewing the great variation in the size of the cells at this period.
Between the epiblast and hypoblast are a number of undifferentiated cells.
It shews the bilobed condition of the alimentary vesicle and the fusion of the mesoblast and hypoblast at the caudal vesicle.
Hypoblast cells from the hinder end of a thirty-six hours' embryo, treated with silver nitrate, shewing the regularity and elongated shape of the cells over the embryo and the smaller cells on each side.
The hypoblast is completely formed in the central part of the blastoderm.
The notochord is hardly differentiated, but the complete separation of mesoblast and hypoblast under the primitive groove is clearly shewn.
Point where epiblast and hypoblast are continuous at the mouth of the alimentary involution.
Section nearer the tail; it shews the thickening of the hypoblast to form the notochord (ch').
The space between the mesoblast and the hypoblast has been made considerably too great in the figures of the three sections.
In b the thickening has become completely separated from the hypoblast as the notochord.
Junction between the white-yolk spheres and the hypoblast cells at the passage from the area pellucida to the area opaca.
It illustrates the continuity of the hypoblast with the remaining lower layer cells of the blastoderm.
The liver, like the pancreas, is developed from the duodenum as an outgrowth from the hypoblast lining the enteric tube.
The pancreatic gland is derived from the hypoblast of the enteric tube.
The proximal and dorsal bud grows directly from the hypoblast lining the duodenum immediately beyond the pyloric junction.
The vesicular mucous surface is thus derived from the enteric hypoblastin the same way as the epithelial lining of the bile-ducts and capillaries.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "hypoblast" in a variety of sentences. We hope that you will now be able to make sentences using this word.
