______transforms the egg from a metabolically depressed state to one of extreme vigor, characterized by a sharp increase in respiratory and synthetic activity.
Fertilization
During _______ waves of cell division follow one another almost without pause, subdividing the unmanageably large zygote into progressively smaller cellular units
Cleavage
the stage of development during which the dividing cell mass doubles with each cycle and resembles a mulberry
Morula
In time the cleaving embryo develops a central cavity or _____ and enters the blastula stage.
Blastocoel
In many species the _______ of the egg is not homogeneous
cytoplasm
In the normal embryo a cleavage division consists of _______ followed by _________
nuclear division (karyokinesis); cell division (cytokinesis)
_______ occurs after karyokinesis
Cytokinesis
The ________ forms first in the region of the cortex nearest to the mitotic spindle and then moves around the cell
cleavage furrow
the position of the cleavage furrow is established by the time of ______
Anaphase
These are composed of microtubules, are the structures that interact with the cell cortex to stimulate the formation of the cleavage furrow
Asters
The cells that arise from cleavage are known as
Blastomeres
The type of cleavage characterized by the complete division of cells, is called
holoblastic cleavage
eggs with small amount of yolk; evenly distributed; vertebrates
oligolecithal eggs
eggs with moderate amounts of yolk; tend to be concentrated toward the vegetal pole, thereby displacing the nucleus toward the animal pole; vertebrates
mesolecithal eggs
eggs with large amount of yolk; commonly displaces the embryo-forming cytoplasm into a small disk on one edge of the ovum; vertebrates
telolecithal eggs
Relatively small animals that need to produce immense numbers of eggs to ensure propagation of the species (e.g., the sea urchin)
True
The first cleavage division cuts the egg in half from the ___________ to the _________ in the plane of the metaphase plate of the dividing nucleus.
animal pole; vegetal pole
eggs that are on the center of oocyte; in aquatic and terrestrial arthropods
Centrolecithal
Cleavage in Amphioxus
the eggs of sea urchins are _____
oligolecithal
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.
True
The fourth cleavage of sea urchin eggs, results in the __________
production of three distinct types of blastomeres
a blastomere of medium size
mesomere
a blastomere of large size; occur in the hemisphere of telolecithal egg containing the vegetal pole
macromere
any of the small cells formed by unequal cleavage of a fertilized ovum
micromere
Formation of _________ is dependent on the presence of cytoplasm located at the vegetal pole; without vegetal pole, micromeres do not form
micromeres
Five territories of sea urchin embryo
1. prospective ectoderm
2. prospective aboral ectoderm
3. prospective skeletogenic mesoderm
4. vegetal plate
5. small micromeres
During the entire period of _______, the embryo is enclosed in the fertilization membrane
cleavage
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.
Blastula
By about the tenth cleavage cycle, the blastomeres form ________which penetrate the hyaline layer and extend into the perivitelline space.
motile cilia
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
True
The final major event during the blastula phase presages the sweeping changes that will occur at ________
gastrulation
The former micromeres will make their way into the blastocoel and will form the __________
primary mesenchyme
The first cleavage division in __________ begins at the animal pole and bisects the gray crescent
amphibians
The __________ begins at the animal pole, with its plane perpendicular to that of the first cleavage plane
second division in amphibians
The third cleavage plane in division in AMPHIBIANS is horizontal and passes nearer to the animal pole, dividing the embryo into _________________.
four smaller blastomeres at the animal hemisphere and four larger blastomeres at the vegetal pole
In amphibians, an embryo between the 16- and 64-cell stages is commonly called a _____
morula
After the morula stage in AMPHIBIANS, a _____________appears in the animal hemisphere above the mass of yolk
prominent cavity (blastocoel)
The amphibian blastula can be conveniently subdivided into three main regions:
1. A region around the animal pole, roughly including the cells forming the roof of the blastocoel. These cells correspond roughly to the future ectodermal germ layer.
2. A region around the vegetal pole, including the large cells in the interior which constitute the yolk mass. These are the future endodermal cells.
3. A marginal ring of cells in the subequatorial region of the embryo, including the region of the gray crescent. Cells of this zone normally form the embryonic mesoderm.
the period in amphibian embryo where the main activity is the increase in the number of cells
pregastrulation
__________ in the amphibian blastula forms in an equatorial ring between the prospective ectoderm and endoderm
mesoderm
(Amphibians) Cells of the animal hemisphere above the blastocoel normally form ________
ectoderm
Cleavage in _____ eggs occurs as the egg is passing down the oviduct
bird/avian eggs
In _______, by the time the egg is laid, early gastrulation has already begun
birds
The first cleavage furrow in birds begins to appear near the center of the blastodisk during __________.
late anaphase of the first mitotic division after fertilization
The sequence of ______- cleavage is not always regular, and after about the third cleavage division it is not synchronous.
avian
outer layer of cells that forms the wall of blastula
blastoderm
the embryo-forming portion of an egg with discodial cleavage usually appearing on the upper surface of the yolk mass
blastodisc
In birds, when the embryo contains about 100 cells, the blastoderm is underlain by a _______
subgerminal cavity
The central portion of the blastoderm, thinned out by the shedding of cells and underlain by the subgerminal cavity
area pellucida
it surrounds are pellucida; the region where the cells of the blastoderm still abut directly onto the yolk
area opaca
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 ______
primary hypoblast
outer layer of the blastoderm
epiblast
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.
primary hypoblast
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
epiblast
Mammals produce extremely small eggs with _________
almost no yolk
In __________, early cleavage divisions are practically unmodified mitoses, or in more technical terms, equal holoblastic cleavage of an isolecithal egg
mammals
Cleavage stages in the pig embryo
Cleavage in mammals is much slower than it is in most other vertebrates
True
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
compaction
_________ prevent the free exchange of fluid between the inside and the outside of the embryo, allowing the accumulation of fluid inside the embryo
Tight Junctions
_________couple all the blastomeres of the compacted embryo and permit the exchange of ions and small molecules from one cell to the next.
Gap junctions
In mammals, by the 16-cell stage the embryo, still enclosed in the zona pellucida, is said to be in the ______ stage
morula
(Mammals) In the morula the internal secretion of fluid by the blastomeres leads to the formation of a well-defined central cavity called the ______
blastocoel or blastocyst cavity
The first is a rapid enlargement of the blastocyst cavity; the second is the emergence of distinctly different types of cells within the embryo.
Two stages that mark the transition from morula to blastocyst
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.
zona pellucida
outer wall of the blastocyst
trophoblast
A is
Morula stage
B is
Blastocyst stage
In both _________ initial control of polarity appears to reside in the vegetative regions of the embryos
amphibians and birds
In birds, early control of polarity resides in the primary hypoblast.
true
the _______________ shows few signs of polarity until well into the stage of the blastocyst
mammalian inner cell mass
egg fragments
merogones
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
synthesis of mRNA and heterogeneous nuclear RNA
the synthesis of __________ is prominent during cleavage
histones
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
Cleavage stage
As the blastula becomes more mature, its activities are increasingly directed toward making preparations for gastrulation.
true