MSCI345 Introduction to the Bilateria

the creation of an egg cell

they are usually larger and contain a lipid-rich substance called yolk, which provides additional nutrients and calories to feed the embryo until it can obtain those things in some other way.

The yolky, lower portion of the egg cell

the less yolky top end of the egg cell

occurs when the cell membranes of a sperm cell and an egg cell fuse together, and the egg cell draws the sperm’s DNA inside

organisms compensate for this problem by releasing millions or even billions of sperm when they spawn.

If an egg cell receives DNA from more than one sperm cell, the result (called polyploidy), causes abnormal cell division in animals. organisms compensate for this problem via polyspermy block

as a sperm fuses with the egg surface, an almost instantaneous wave of electrical activity passes over the egg surface and sets up a barrier that makes it difficult for any other sperm cells to fuse with that egg cell

slower, but more impenetrable changes in the chemical structure of the cell surface following fast polyspermy block

the zygote undergoes a series of rapid cell divisions (without growth) that subdivide the zygote into a mass of much smaller cells. This process of dividing or “cleaving” the embryo through a series of divisions

splits the zygote into two roughly-identical daughter cells. In most species, this first cell division occurs along a plane containing the sperm entry point and the animal-vegetal axis.

After 1st cleavage, the embryo contains 2 cells and is said to be at the ” “stage of embryo development.

occurs on a plane that also contains the animal-vegetal axis, but this cleavage plane is oriented approximately 90-degrees to the first cleavage plane. This would be analogous to slicing Earth from the 90 degrees longitude line all the way through to the 270 degrees longitude line.

2nd cleavage cuts each of the initial 2 cells in half, resulting in the “ “ stage of embryo development.

generally occurs at right angles to both of the first two cleavage planes This is analogous to separating the earth into Northern and Southern hemispheres by slicing through the equator.

The 3rd cleavage cuts each of the 4 previous cells in half, resulting in a total of 8 cells in the embryo, 4 in the animal (upper) half and 4 in the vegetal (lower) half of the embryo. At this point, the embryo is said to have reached the “ “ stage of development.

each of the 4 cells in the “northern hemisphere” sits directly above (i.e., due north of) one of the 4 cells in the southern hemisphere.

the northern hemisphere rotates (usually westward) 45 degrees relative to the southern hemisphere, so that the 4 northern cells come to rest in the furrows between the 4 southern hemisphere cells. In animals with this cleavage, this pattern of 45-degree rotations continues for a number of subsequent cleavage divisions, resulting in a twisting, spiral-staircase-like arrangement of cells in the young embryo.

In some animals, the developmental fate of the daughter cells formed by cleavage is completely determined from the moment of cleavage and cannot be changed after that. If you take one of these embryos at the two-cell stage and separate the two cells, each cell will start growing into just half an animal! That, of course, is not viable, and the two half-embryos will die before very long.

In other animals, the cells of the early embryo can (for a short time and under certain circumstances) shift roles and become different tissues or organs than they normally would have become. This enables the embryo to compensate for loss or damage to one of the cells

When cleavage is finished, or nearly so, the resulting embryonic stage. In most species, it is a hollow, fluid-filled ball made of hundreds of cells.

Cells on the outer surface of the blastula form an epithelial layer or “ “ . It is a specialized tissue in which the cells adhere to each other tightly enough to form a barrier to movement of most molecules between the inside and outside of the embryo. Having it gives the embryo greater control over its internal fluid composition, because it can use membrane channel proteins and other means to regulate what crosses it and thereby control what moves in or out of the embryo.

The fluid-filled, hollow space inside the blastula

an important developmental process that occurs in all animals except sponges. The process begins when one end of the hollow blastula begins to indent, poking into the fluid-filled blastocoel. This inpocketing usually starts at the vegetal end of the embryo.

During and after gastrulation, the epithelial layer that remains on the outside of the embryo

The epithelial layer that has been folded into the interior of the embryo and now lines the gut. In cnidarians it is the gastroderm

In cnidarians and ctenophores, the region between the ectoderm and endoderm/gastroderm is generally filled with a mostly non-cellular matrix of proteins and water

But in most other animals (i.e., pretty much every animal that isn’t a sponge, cnidarian, or ctenophore) the space between the ectoderm and endoderm gets filled with cells to form a third germ layer

Most triploblastic animals have a hollow body cavity in which the main organs reside. If that body cavity is lined with mesodermal tissue

the coelom forms by a split in the mesoderm

the coelom forms from mesodermal outpocketings from the side of the gut

the body cavity is not lined with mesoderm, but instead is the remains of the early embryonic blastocoel

If the animal has neither a true coelom nor a pseudocoelom and is, instead, just solid tissue between the gut and outer surface, then it is described as being an “ “

an enormous clade that includes just about all animals except sponges, cnidarians, and ctenophores. Major sub-groups within this group are distinguished, in large part, by differences in their embryonic development.

An animal with bilateral symmetry typically has a left and right side that are roughly mirror images of each other. They also have a doral (usually top) and ventral (usually bottom) surface, and an anterior (front) and posterior (back) end.

• bilateral symmetry

• triploblastic

• organ-system level of organization

the bilaterians do form a ” “ during gastrulation. That means their gut has an opening at both ends, so food can pass through it in one direction only. This is important; it allows for specialization of different parts of the gut for different functions, like different sections of an assembly line in a factory.

• the mouth forms where the inpocketing of gastrulation begins, and the anus doesn’t appear until the newly forming gut connects to the other side of the embryo.

• tend to have mosaic development

• form coeloms through schizocoely

• the anus forms where the inpocketing of gastrulation begins, and the mouth doesn’t appear until the newly forming gut connects to the other side of the embryo.

• tend to have regulative development

• form coeloms through enterocoely

• tend to have radial cleavage during early embryonic development.

united by having an exoskeleton that must be shed or molted through a process known as ecdysis

The majority of marine animal species -- even those that spend their adult lives as sessile benthic organisms -- pass through a “ “

A ball of cells, with one hemisphere made of small cells that have flagella and the other half made of relatively large cells without flagella. Swims slowly and smoothly; This is an early larval stage of many types of sponges.