Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1
Cleavage and Germ Layers.
One cell, the zygote, is where a tapeworm, a Hydra and a human all begin. The start is the same for every one of them: the cell keeps dividing. How many layers the embryo ends up with is what first splits the animal kingdom into two camps.
1. The zygote divides: cleavage
The single-celled zygote of a sexually reproducing animal divides again and again by mitosis and builds a many-celled embryo. This process is called cleavage.
The kind of cleavage depends on how much yolk the egg carries: complete, or holoblastic, and partial, or meroblastic. During cleavage the end of the egg that holds the yolk is the vegetal pole, and the end that holds the nucleus is the animal pole.
Run the divisions one at a time, from zygote to gastrula. At eight cells, switch between radial and spiral: the difference is all in the third division.
step throughwatch the third division
From one cell to layers
2. Change the plane, change the cleavage
By the plane of division, cleavage is of three types: radial, bilateral and spiral.
In radial cleavage the division planes always cut the zygote radially and evenly; the example is Arthropoda, such as the grasshopper. In bilateral cleavage the first two divisions look just like radial cleavage. Later divisions then run across the midline and give two rows of four cells each. Chordata show this type.
In spiral cleavage, at the third division the blastomeres of the animal pole shift slightly in a spiral against those of the vegetal pole. Annelida and Mollusca show this type. So when you get a clue, ask first: what happened at the third division?
sort themThe difference shows at the third division
Which cleavage?
Pick up a clue, then drop it in a cleavage box.
Right. Next one.
Not yet. In radial cleavage the planes are always radial and even. In bilateral, the divisions after the second run across the midline. In spiral, the third division shifts the cells in a spiral.
All in place. Radial is Arthropoda, bilateral is Chordata, spiral is Annelida and Mollusca.
3. From zygote to gastrula
Cleavage turns the zygote into small cells called blastomeres. The cells then pass through the solid morula, the hollow blastula, and finally the gastrula stage. Solid first, hollow later: this is exactly where morula and blastula get swapped.
in orderSolid first, hollow later
Zygote to gastrula
Where does it start? With a single cell.
Yes. Then?
Not yet. Before cleavage there is only the zygote. Before the morula, cleavage makes blastomeres. Cells are a solid morula before the cavity opens. A hollow blastula must exist before layers form.
Right order. The gastrula is two-layered or three-layered, and that layer count splits animals into diploblastic and triploblastic.
Germ layers play a special role in the early classification of animals. The gastrula is two-layered or three-layered, and on that basis animals fall into two groups: diploblastic and triploblastic.
4. Diploblasts and triploblasts
Animals whose gastrula has its cells arranged in two layers, ectoderm and endoderm, are diploblastic. Between the two layers lies mesoglea, a non-cellular, jelly-like layer. Cnidaria, for example Hydra, are diploblastic.
Animals whose gastrula has three cell layers are triploblastic: ectoderm outside, mesoderm in the middle, endoderm inside. Every animal from Platyhelminthes (the tapeworm Taenia solium) up to Chordata (human, Homo sapiens) is triploblastic.
The various organs of the animal body arise from these germ layers. Now name the layers of Figure 1.1.
Figure 1.1Jelly in the middle for diploblasts, a cell layer for triploblasts
Name the layers
Drawn by hand after the book's Figure 1.1, not to scale.
5. The two groups compared
The differences from the book's table are laid out in one place. Tap any cell and the note for that row appears below. To start, ask yourself: which group is Hydra in?
gridWhich group is Hydra in?
Two groups compared
property
diploblastic
triploblastic
germ layers
two
three
mesoglea
present
absent
nematocyst
present
absent
polyp, medusa
present
absent
body cavity
coelenteron
coelom
tissues, organs
cannot
can
Germ layers. Two layers make a diploblast: ectoderm and endoderm. Three make a triploblast: ectoderm outside, mesoderm in the middle, endoderm inside.
Mesoglea. This non-cellular layer lies between the ectoderm and endoderm of a diploblast. Triploblasts have no mesoglea. Hydra belongs to Cnidaria, so it is diploblastic.
Nematocyst. Present in diploblasts, absent in triploblasts.
Polyp and medusa. These stages, both or one of them, occur in diploblasts and not in triploblasts.
Body cavity. The body cavity of a diploblast is the gastrovascular cavity, or coelenteron. That of a triploblast is the coelom.
Tissues and organs. In triploblasts the germ-layer cells form tissues, organs and organ systems. The book's table says the cells of diploblasts cannot form tissues or organs.
The whole idea in one line: germ layers are the three basic layers formed during gastrulation, ectoderm, mesoderm and endoderm, from which the animal's organs and tissues arise.
6. Check yourself
Start with the first question.
check yourselfpick one answer
Question 1
From where do the various organs and parts of the animal body arise?
The coelom is the cavity between body wall and gut. The organs arise from the germ layers.
Right. The ectoderm, mesoderm and endoderm formed during gastrulation give rise to the animal's organs and tissues.
A segment is a repeated part of the body. Organs arise from germ layers, not from segments.
The notochord is the dorsal supporting rod of chordates. It is not the source of all organs; the germ layers are.
Think the second one through.
check yourselfpick one answer
Question 2
In which phylum is spiral cleavage seen?
The book gives Arthropoda as the example of radial cleavage, not spiral.
Chordata show bilateral cleavage: later divisions run across the midline to give two rows of four cells.
Right. At the third division the animal-pole blastomeres shift in a spiral in Annelida and Mollusca.
The book does not give Cnidaria as an example of spiral cleavage. Spiral cleavage is shown by Annelida and Mollusca.
The last is in the exam's style.
check yourselfpick one answer
Question 3
What is the end of the egg that holds the yolk called?
The gastrula is an embryonic stage. The yolk end of the egg is the vegetal pole.
The end holding the nucleus is the animal pole. The yolk lies at the opposite end.
The morula is a stage of cleavage, a solid ball of cells, not a name for an end of the egg.
Right. The end of the egg holding the yolk is the vegetal pole.
The whole note on eight cards.
cut from the notetap to turn over
Eight cards
QuestionWhat is cleavage?
AnswerThe mitotic division of the single-celled zygote of a sexually reproducing animal into a many-celled embryo.
QuestionBy division plane, how many types of cleavage are there?
AnswerThree: radial, bilateral and spiral.
QuestionWhat happens at the third division of spiral cleavage, and in which two phyla is it seen?
AnswerAnimal-pole blastomeres shift slightly in a spiral; Annelida and Mollusca.
QuestionWhat is the order of stages from zygote to gastrula?