EMBRYOL Chapter 5 - Cleavage and Formation of the Blastula

______transforms the egg from a metabolically depressed state to one of extreme vigor, characterized by a sharp increase in respiratory and synthetic activity.

During _______ waves of cell division follow one another almost without pause, subdividing the unmanageably large zygote into progressively smaller cellular units

the stage of development during which the dividing cell mass doubles with each cycle and resembles a mulberry

In time the cleaving embryo develops a central cavity or _____ and enters the blastula stage.

In many species the _______ of the egg is not homogeneous

In the normal embryo a cleavage division consists of _______ followed by _________

_______ occurs after karyokinesis

The ________ forms first in the region of the cortex nearest to the mitotic spindle and then moves around the cell

the position of the cleavage furrow is established by the time of ______

These are composed of microtubules, are the structures that interact with the cell cortex to stimulate the formation of the cleavage furrow

The cells that arise from cleavage are known as

The type of cleavage characterized by the complete division of cells, is called

eggs with small amount of yolk; evenly distributed; vertebrates

eggs with moderate amounts of yolk; tend to be concentrated toward the vegetal pole, thereby displacing the nucleus toward the animal pole; vertebrates

eggs with large amount of yolk; commonly displaces the embryo-forming cytoplasm into a small disk on one edge of the ovum; vertebrates

Relatively small animals that need to produce immense numbers of eggs to ensure propagation of the species (e.g., the sea urchin)

The first cleavage division cuts the egg in half from the ___________ to the _________ in the plane of the metaphase plate of the dividing nucleus.

eggs that are on the center of oocyte; in aquatic and terrestrial arthropods

the eggs of sea urchins are _____

The first two divisions of sea urchin eggs are meridional (from animal pole to vegetal pole), and the third is equatorial, producing an eight-cell embryo with blastomeres of almost equal size.

The fourth cleavage of sea urchin eggs, results in the __________

a blastomere of medium size

a blastomere of large size; occur in the hemisphere of telolecithal egg containing the vegetal pole

any of the small cells formed by unequal cleavage of a fertilized ovum

Formation of _________ is dependent on the presence of cytoplasm located at the vegetal pole; without vegetal pole, micromeres do not form

Five territories of sea urchin embryo

During the entire period of _______, the embryo is enclosed in the fertilization membrane

As the embryo goes into the seventh and eighth cleavage cycles, the central cavity (blastocoel) becomes well established and the embryo has entered the _____ phase.

By about the tenth cleavage cycle, the blastomeres form ________which penetrate the hyaline layer and extend into the perivitelline space.

The next morphological change after the cells of blastula secrete into the perivitelline space is the formation of a tuft of long, nonmotile cilia at the animal pole of the blastula

At about the same time, the region around the vegetal pole flattens to form the micromere-containing vegetal plate

The final major event during the blastula phase presages the sweeping changes that will occur at ________

The former micromeres will make their way into the blastocoel and will form the __________

The first cleavage division in __________ begins at the animal pole and bisects the gray crescent

The __________ begins at the animal pole, with its plane perpendicular to that of the first cleavage plane

The third cleavage plane in division in AMPHIBIANS is horizontal and passes nearer to the animal pole, dividing the embryo into _________________.

In amphibians, an embryo between the 16- and 64-cell stages is commonly called a _____

After the morula stage in AMPHIBIANS, a _____________appears in the animal hemisphere above the mass of yolk

The amphibian blastula can be conveniently subdivided into three main regions:

the period in amphibian embryo where the main activity is the increase in the number of cells

__________ in the amphibian blastula forms in an equatorial ring between the prospective ectoderm and endoderm

(Amphibians) Cells of the animal hemisphere above the blastocoel normally form ________

Cleavage in _____ eggs occurs as the egg is passing down the oviduct

In _______, by the time the egg is laid, early gastrulation has already begun

The first cleavage furrow in birds begins to appear near the center of the blastodisk during __________.

The sequence of ______- cleavage is not always regular, and after about the third cleavage division it is not synchronous.

outer layer of cells that forms the wall of blastula

the embryo-forming portion of an egg with discodial cleavage usually appearing on the upper surface of the yolk mass

In birds, when the embryo contains about 100 cells, the blastoderm is underlain by a _______

The central portion of the blastoderm, thinned out by the shedding of cells and underlain by the subgerminal cavity

it surrounds are pellucida; the region where the cells of the blastoderm still abut directly onto the yolk

At about the time the egg is laid (in birds), individual cells or aggregates of cells shed from the lower surface of the blastoderm by a process of polyingression coalesce to form a thin disklike layer called the ______

outer layer of the blastoderm

The __________, which ultimately forms extraembryonic endoderm, possesses an inherent polarity which it confers on the embryo proper, which is represented at this stage by the early epiblast.

The _________ is considered the equivalent of the animal hemisphere, and the primary hypoblast shares many common properties with the vegetal hemisphere of the amphibian embryo

Mammals produce extremely small eggs with _________

In __________, early cleavage divisions are practically unmodified mitoses, or in more technical terms, equal holoblastic cleavage of an isolecithal egg

Cleavage in mammals is much slower than it is in most other vertebrates

A critical stage that takes place at the eight-cell stage in the mouse.

During this, the blastomeres flatten and become tightly joined so that they cannot become distinguished from one another with the light microscope

_________ prevent the free exchange of fluid between the inside and the outside of the embryo, allowing the accumulation of fluid inside the embryo

_________couple all the blastomeres of the compacted embryo and permit the exchange of ions and small molecules from one cell to the next.

In mammals, by the 16-cell stage the embryo, still enclosed in the zona pellucida, is said to be in the ______ stage

(Mammals) In the morula the internal secretion of fluid by the blastomeres leads to the formation of a well-defined central cavity called the ______

The first is a rapid enlargement of the blastocyst cavity; the second is the emergence of distinctly different types of cells within the embryo.

The early mammalian blastocyst remains enclosed within the _________, but the overall size of the embryo increases to some extent because of the accumulation of fluid.

outer wall of the blastocyst

A is

B is

In both _________ initial control of polarity appears to reside in the vegetative regions of the embryos

In birds, early control of polarity resides in the primary hypoblast.

the _______________ shows few signs of polarity until well into the stage of the blastocyst

egg fragments

As early as the third or fourth cleavage division, the ______________ begins.

New rRNA and tRNA appear around the time of the blastula. Overall, there is a steady increase in embryonic RNA synthesis until the blastula stage, at which point it begins to level off

the synthesis of __________ is prominent during cleavage

the increase in cell number is the dominant manifestation of _____a simple phase of development during which the embryo is simply increasing its number of cells

As the blastula becomes more mature, its activities are increasingly directed toward making preparations for gastrulation.