The small *but whole* gastrula and pluteus that actually *do* result.
You know, from the second lecture, what a gastrula of our sea-urchin is.
Small but whole gastrula that results by a process of regulation from the parts of a bisected gastrula.
The gastrula is formed in this case by invagination.
The two-layered young stage of higher creatures, when it has a free-swimming existence, is called a Gastrula (Fig.
For the young sponge is at first a little free-swimming, two-layered animal of the type which has been described above as the gastrula larva.
A very wide-spread form of larva, more advanced in its structure than the little Gastrula that has been already named, has received the name of Trochosphere or Wheel-ball (Fig.
By the conversion of the one-layered blastula into two layers of cells, the Gastrula stage of the embryo is attained.
The Gastrula consists of two layers of cells surrounding a central cavity, which is the Archenteron, or the body-cavity that will hold the intestines.
From the fact that a gastrula stage is very commonly formed when segmentation has been completed, they assumed that all germ-layered animals were descended from an ancestral Gastræa.
The gastrula stage was the palingenetic repetition of the ancestral form of all Metazoa, the Gastræa.
These were the monerula, cytula, morula, blastula, and gastrula (see Fig.
Applied to a gastrula when the blastorope does not entirely up.
The primitive enteron or undifferentiated digestive sac of a gastrula or other embryo.
The unsymmetrical gastrula or pseudo-gastrula which is common to them all is, I believe, to be explained by the form of the vertebrate body.
No satisfactory phylogenetic explanation of the Mammalian gastrula by epibole has in my opinion as yet been offered.
It would seem to follow from this that a delaminate gastrula has here been a secondary result of an abbreviation in the development.
The generative cells appear during the gastrula stage, as two large cells with conspicuous nuclei, which are placed in the hypoblast lining the archenteron, at the pole opposite the blastopore.
This third well-marked stage is called the gastrula stage; and it is thought to occur either typically or in some modified form in the development of all metazoa, or many-celled animals.
In all the fishes the development of the embryo goes on within the egg long after the gastrula stage is passed, and until the embryo becomes a complex body, composed of many differing tissues and organs.
The next stage is the conversion of the blastula into a double-walled cup, known as a gastrulaby the pushing in of one side.
In Petromyzon and the Dipnoans blastula and gastrula stages result, which, though differing in some particulars from the corresponding stages of the lancelet, may yet readily be compared with them.
Developing, in the case of multicellular organisms, from the same embryonic systems into which the secondary unit (gastrula or plant enbryo) differentiates.
The process of invagination, in embryonic development, by which a gastrulais formed.
By the perforation of the resultant two-walled vesicle, a gastrula results similar to that formed by the process of invagination.
In the gastrula stage, the segmentation cavity in which the mesoblast is formed lies between the entoblast and ectoblast.
With what lower type has thegastrula been compared?
The gastrula in section, anterior end to the right.
All animals that have little yolk, and start early in life for themselves, pass through a gastrula stage, substantially the same as this of amphioxus.
We naturally assume, from what we have learnt, that the next stages will be the formation of a hollow blastosphere, invagination, a gastrula forming mesoblast by hollow outgrowths from the archenteron, and so on.
Such a gastrula, formed mainly by overgrowth of the epiblast, is called an epibolic gastrula, as distinguished from the invaginate gastrula of amphioxus.
The young amphioxus has, at this stage, which is called the gastrula stage, a curious parallelism with such a lowly form as the Hydra of our ditches.
This idea is supported by the fact that the first development--in the sea urchin to the gastrula stage inclusive--is independent of the nucleus, which is the bearer of the Mendelian factors.
From the occurrence of the gastrula in all the divisions of the animal kingdom, we may conclude that it is a temporary phase, inevitable in the growth of animals.
From causes of which we are ignorant two, instead of one, gastrula stages may arise at separate regions of the germinal layer of the egg.
Successive Stages in the Development of the Ovum to the Gastrula Stage] =The appearance of life.
From the gastrula type the higher subdivisions of animal life probably diverged, as separate branches from a common trunk.
All forms of animal life, from the coelenterates to the mammals, follow the same path in their embryological development as far as the gastrula stage, but here their paths widely diverge, those of each subkingdom going their own separate ways.
This cup-stage is called the gastrula stage; the outer wall of the gastrula is the ectoderm, and its inner the endoderm; while its cavity is the enteron, and the opening to the exterior the blastopore.
Defn: The process of invagination, in embryonic development, by which a gastrulais formed.
Defn: Developing, in the case of multicellular organisms, from the same embryonic systems into which the secondary unit (gastrula or plant enbryo) differentiates.
Like the latter, the human gastrula and that of all other mammals must be regarded as the ontogenetic reproduction of the phylogenetic form that we call the Gastraea, in which the whole body is nothing but a double-walled gastric sac.
The pure type of the original gastrula (or archigastrula, Figure 1.
This is done in just the same way as the invagination of the blastula, which gives rise to the gastrula in the amphioxus (Figure 2.
In reality, the whole body of the gastrula is merely a "primitive gut.
In all these stems the gastrula recurs in the same very simple form.
The formation of these very important organs takes place in the Ascidia-gastrula in precisely the same way as in that of the Amphioxus.
In the Amphioxus the earliest and most important embryonic processes take place so rapidly that the blastula is formed in four hours, the gastrula in six, and the typical vertebrate form in twenty-four.
From the epidermis of the gastrula a medullary tube is formed on the dorsal side, and, between this and the primitive gut, a chorda; these are the organs that are otherwise only found in Vertebrates.
From thegastrula is first formed the spherical embryonic vesicle filled with fluid (gastrocystis, Figure 1.
In the further course of its rapid development the roundish bell-gastrula becomes elongated, and begins to flatten on one side, parallel to the long axis.
The clear path from the stem-cell to the gastrula represents the first section of our human stem-history (Chapters 1.
Probably it differed from the existing gastrula only in one essential point, though extinct millions of years ago.
It is the first stage in postembryonic development, and follows the gastrula stage.
What invertebrates resemble thegastrula in body plan?
Slides or diagrams, showing various stages in the development of some animal through the gastrula stage.
Contrast the blastula andgastrula as to number of cavities, number of cell layers, number of external openings.
We may therefore compare the hydra or gastrula to a little portion of the lining of the human mid-intestine covered with a little flake of epidermis.
Both the blastosphere andgastrula often swim freely by flagella.
A) Gastrula of a very simple primitive-gut animal or gastraead (gastrophysema).
In the bell-gastrula of the amphioxus and in the hooded gastrula of the lamprey and the frog the germinal layers are found to be closed tubes or vesicles from the first.
The distinctive form of gastrula that comes of it is the "vesicular gastrula" (Perigastrula).
The cleavage and formation of the gastrula take place in the oviduct.
It is clear that the important features which distinguish the discoid gastrula from the other chief forms we have considered are determined by the large food-yelk.
I regard the gastrula as the most important and significant embryonic form in the animal world.
The authors who regard this embryonic form as the primarygastrula of the placental conceive the outer layer as the ectoderm and the inner as the entoderm.
As the discoid gastrula grows round the large globular yelk, and the permanent gut then separates from the outlying yelk-sac, we find all the processes which we have shown (diagrammatically) in Figure 1.
This gastrula theory, now generally accepted, is one of Haeckel's two great fundamental contributions to the evolution philosophy with the history of which his life work is so intimately linked.
A certain proof of this is furnished by the amphioxus, which, in spite of its blood relationship to man, still passes through the stage of the gastrula with a simple intestine and a double intestinal wall.
This gastrula is the simplest form of the "person," and the two germinal layers are its sole organs.
Of the four stems of the coelenteria (which have only a ventral opening and no gut-cavity) the gastraeads remain at the gastrula stage, and the sponges are formed by multiplication of the same stems of gastraeads.
In Cordylophora the embryo is set free at the parenchymula stage as a planula which fixes itself and develops into a polyp, both gastrula and actinula stages being suppressed.
The gastrula has now become an actinula, which may be termed the distinctive larva of the Cnidaria, and doubtless represents in a transitory manner the common ancestor of the group.
After thegastrula stage, which is found as a developmental stage in all Enterozoa, the embryo of the Hydrozoa proceeds to develop characters which are peculiar to the Coelenterata only.
On the central gastrula cavity of the sponge becoming placed in communication with the external water, the hypoblast cells lining it become ciliated afresh (fig.
The blastopore would however be situated dorsally, a position which it does not occupy in any gastrula type so far dealt with.
The bilateral Auricularia is developed from a slightly elongated gastrula with an uniform covering of cilia.
In Nephelis however the segmentation is very abnormal, and the formation of the germinal layers cannot easily be reduced to an invaginate gastrula type, though probably it is modified from such a type.
The central cavity of the gastrula is lined by hypoblast cells, but the oral opening, which leads by a narrow passage into the gastric cavity, is lined by epiblast cells.
Immediately after the completion of the gastrula a remarkable series of phenomena takes place.
The development continues, however, to be normal up to the gastrula stage, at any rate, if three or even four spermatozoa enter the egg almost simultaneously.
In Lumbricus the enteric cavity is formed during the gastrula stage.
The gastrula opening is stated to remain as the permanent mouth, which has a terminal and central position.
In all these cases, however, as was first shewn by Lankester and Selenka, a gastrula becomes formed either by normal invagination as in the case of Paludina (fig.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "gastrula" in a variety of sentences. We hope that you will now be able to make sentences using this word.
