Our next question concerns the distribution of potentiality, when the embryo is developed further than the blastula stage.
Since micromeres and intestine material are present and in their normal position an intestine will grow into the blastula and a whole organism will result.
Then the cilia are formed at the external surface of these cells and the egg begins to swim; we say it has reached the first larval, the so-called blastula stage.
This symmetry is probably due to the following fact: the first cleavage plane goes through that spot where the intestine grows into the blastula cavity.
It looks as if a gradual migration of all the entoderm material from the ectoderm into the entoderm took place during the blastula formation.
The isolated ectoderm cells of a blastula could no longer form an intestine; they were lacking the entoderm material.
Thus in Clathrina blanca the ovum undergoes a regular and total cleavage, resulting in the formation of a hollow ciliated blastula of oval form.
So, also, it must be with ontogeny; here also the central factor in the formation of the adult from the blastula ought to be the formation of the central nervous system, not that of the gut.
Longitudinal section through the blastula of a shark (Pristiuris).
Although the mass of the food-yelk may be very large in the ova of the discoblastic vertebrates, nevertheless in every case a blastulais developed from the morula, as in the holoblastic ova.
The blastula of the lower animals must not be confused with the very different blastula of the mammal, which is properly called the gastrocystis or blastocystis.
Moreover, the segmentation-cavity of the blastula (Figure 1.
Longitudinal section of the blastula of a shark (Pristiurus) at the beginning of gastrulation.
I have in my Natural History of Creation given the name of depula to the remarkable intermediate form which appears at the passage of the blastula into the gastrula.
Formerly this real blastula was generally believed to be equivalent to the embryonic vesicle of the mammal.
This cenogenetic gastrocystis and the palingenetic blastula are sometimes very wrongly comprised under the common name of blastula or vesicula blastodermica.
It is only by this phylogenetic explanation that we can understand the formation and development of the peculiar, and hitherto totally misunderstood, blastula of the mammal.
The spherical blastula that arises from the impregnated ovum of the sagitta is converted by a folding at one pole into a typical archigastrula, entirely similar to that of the Monoxenia which I described (Chapter 1.
The globular wall of the blastula is a real blastoderm, and consists of homogeneous (blastodermic) cells; it is not yet differentiated into the two primary germinal layers.
When the blastula is spherical and not set free, the germ-layer formation is always multipolar, either by immigration or by delamination, i.
Next the blastula gives rise to an internal mass of cells (fig.
When the blastula is oval and free-swimming the inner mass is formed by unipolar immigration from the hinder pole.
The next stage is the conversion of the blastula into a double-walled cup, known as a gastrula by the pushing in of one side.
In a blastula showing complete segmentation the blastomeres of the upper hemisphere are the more finely subdivided.
From the blastula stage to that of the early gastrula, the changes have been but slight; the blastoderm has greatly flattened out as its margins grow downward, leaving the segmentation cavity apparent.
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.
And a similar argument might apply to the small but whole blastulaof Echinus, and to all other cases.
We may formulate this result in the words: the prospective potency of the single cells of a blastula of Echinus is the same for all of them; their prospective value is as far as possible from being constant.
But there is in the blastula no trace of one *part* of the germ becoming different with respect to others of its parts.
The cells which in the above manner have entered the interior of the blastula are to be the foundation of important parts of the future organism; they are to form its connective tissue, many of its muscles, and the skeleton.
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.
The actual ontogenetic development of the gastrula from the blastula furnishes sound evidence as to the phylogenetic origin of the Gastraea from the Blastaea.
In many of the lower animals the blastula is not developed within the foetal membranes, but in the open water.
A real gastrula (a simple bell-gastrula) is formed from theblastula by invagination, in the same way as in the amphioxus.
A pit-shaped depression appears at one side of the sphericalblastula (Figure 1.
These important early embryonic processes take place so quickly in the Amphioxus that four or five hours after fecundation, or about midnight, the spherical blastula is completed.
Moreover, in the animals in which we do not find a real palingenetic blastula the defect is clearly due to cenogenetic causes, such as the formation of food-yelk and other embryonic adaptations.
Then follows the phenomenon of gastrulation, by which one-half of the blastulais invaginated into the other, so as to obliterate the segmentation cavity.
The segmentation or cleavage of the ovum which follows upon fertilization terminates in the achievement of the blastula form, a minute sphere of cells surrounding a central cavity.
The Blastula of most Invertebrata, and even that of Amphioxus, is possessed of fine ciliae, or hair-like processes, the vibrating motion of which causes the whole organism to rotate and advance in the water.
Suggest protozoans that resemble the egg and blastula respectively.
Gastrula, a stage formed from the blastula by pushing in one side of the latter, so as to form a more or less cup-shape structure.
Contrast the blastula and gastrula as to number of cavities, number of cell layers, number of external openings.
Is there any considerable difference between the size of the egg and the size of the blastula and gastrula?
This blastula is formed of superposed layers, each of which gives birth to specialised organs in the embryo.
The blastula differentiates itself into embryonic layers, the ectoderm, endoderm, and mesoderm already mentioned.
The psychological phenomena which we directly observe in the formation of the blastula are partly sensations, partly movements, of this community of cells.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "blastula" in a variety of sentences. We hope that you will now be able to make sentences using this word.
