I. Early development
A.
The
newly fertilized zygote is much larger than a normal body cell
1.
The
zygote divides a number of times until the cells of what is now the embryo are
of normal somatic cell size
a)
This
process is called cleavage
b)
During
this time the embryo does not grow in size
c)
Each
one of the cells of the embryo is called a blastomere
(1)
At
the end of cleavage, the number of cells varies (depending on species) from
1000 to 700,000
B.
Patterns
of cleavage
1.
Cleavage
patterns are affected by
a)
Quantity
& distribution of yolk (Fig. 8-7)
(1)
Yolk
retards the speed of cleavage – this means that different regions of the embryo
may divide at different rates
(a)
Isolecithal
egg – little yolk
(b)
Mesolecithal
egg – moderate amount of yolk
(i)
Gives egg polarity
(a)
End containing yolk is called vegetal pole
(b)
Opposite end is called animal pole – mostly cytoplasm
& little yolk
(c)
In
isolecithal & mesolecithal eggs, cleavage is holoblastic – the
cleavage furrow extends completely through the egg
(d)
Telolecithal
– lots of yolk concentrated at vegetal end
(i)
Cleavage is telolecithal eggs is meroblastic – cleavage
furrows extend only part way through the egg
b)
Genes
controlling the symmetry of cleavage (Fig. 3-6, Lab Manual)
(1)
Radial
cleavage – cleavage furrows are oriented parallel or perpendicular to
animal-vegetal pole axis
(a)
Animals
that undergo radial cleavage are called the deuterostomia
(b)
Includes
echinoderms, hemichordates, & chordates
(2)
Spiral
cleavage – cleavage furrows are oriented obliquely to animal vegetal pole – blastomeres
end up lying in cleavage furrows
(a)
Animals
that undergo spiral cleavage are called the protostomia
(b)
Found
in annelids, nemerteans, tubellarian flatworms, molluscs (except cephlopods),
& several other invertebrate phyla
(3)
Protostome
vs. deuterostome distinction is important in other aspects of development
c)
Some
other groups like insects (protostomes) & mammals (deuterostomes) have
unique cleavage patterns (described in text)
II. Blastulation (Fig 1; similar to Fig. 3-9, Lab Manual)
A.
Following
cleavage, the embryo is a mass of cells called a blastula
1.
In
many animals, the cells surround a fluid-filled cavity called a blastocoel
III. Gastrulation
A.
Gastrulation
converts a spherical blastula into a complex structure with 3 layers
B.
Pattern
of gastrulation varies tremendously depending on the amount of yolk
1.
Simple
in non-yolky embryos, complex in yolk-laden embryos
C.
We’ll
use the relatively simple gastrulation of the frog as our example
1.
Gastrulation
begins with the invagination of cells into the interior of the blastula
2.
The
invaginating cells move into the blastocoel & reduce its size (Fig. 1g-i)
3.
During
invagination, a new cavity is formed, the archenteron
a)
The
archenteron will form the gut
4.
The
opening of the archenteron is called the blastopore
a)
In
protostomes, the blastopore becomes the mouth, the anus will form secondarily
(Fig. 8‑9)
b)
In
deuterostomes, the blastopore becomes the anus, the mouth forms secondarily
5.
Note
that the blastocoel is being obliterated during gastrulation
a)
In
some animals, the blastocoel will persist – more on that later
6.
The
embryo now has 2 cell layers, the outside layer is called ectoderm &
the cells lining the archenteron are called endoderm
a)
Most
animals will form a cell layer between the ectoderm and endoderm, the mesoderm
(Fig. 1j)
(1)
Cnidaria
& Ctenophora do not form mesoderm
IV. Coelom formation (Fig. 9-12)
A.
Many
animals form a cavity inside the body called a coelom
1.
Animals
that do not form a coelom are called acoelomate
a)
Acoelomates
include flatworms & nemerteans
b)
The
area between the body wall & the gut is filled by a network of cells a
parenchyma (mesodermal origin)
B.
Animals
with a body cavity are divided into two large groups depending on how the body
cavity is formed, pseudocoelomates & coelomates
1.
In
pseudocoelomates, the blastocoel persists after gastrulation & forms the
pseudocoelom
a)
The
pseudocoelom is a perfectly good body cavity, but is called a “pseudocoelom”
because the cavity isn’t lined by a mesodermal peritoneum (gut isn’t lined
[surrounded] by mesoderm)
b)
Organs
derived from the mesoderm are found within the pseudocoelom
c)
Pseudocoelomates
include: nematodes, rotifers,
gastrotrichs
2.
Coelomates
form a “true coelom” in one of 2 ways (Fig. 9-13)
a)
Protostomes
form their coelom via a process called schizocoelous development
(1)
Mesoderm
forms from cells near lips of blastopore
(2)
Mesoderm
splits (schizo = split) to form coelom
b)
Most
deuterostomes form their coelom via enterocoelous development
(1)
Mesoderm
forms from outpocketings of the archenteron
c)
In
both schizocoelous & enterocoelous development the coelom enlarges to obliterate
the blastocoel
(1)
The
mesodermal peritoneum will now line both the body wall & gut, with a double
layer mesentery between the body wall & gut
(a)
The
mesoepithelium you viewed in lab was this mesentery
(2)
Note
that the organs do not lie within the coelom itself
(a)
All
mesodermal organs are covered by the peritoneum (simple squamous epithelium)
d)
Vertebrates
are deuterostomes that do not form the coelom via enterocoelous development
(1)
Vertebrates
produce their coelom via a secondarily derived schizocoelous development
(a)
Necessary
to deal with large amount of yolk in ancestral vertebrate eggs
V. Animal symmetry
A.
When
we examine animals, we note two general forms of “life styles” – either they
actively move in specific directions or they don’t
1.
These
differences in “life styles” produce differences in body form
B.
Animals
that do not move in specific directions tend to be sessile or pelagic
a)
Sessile
= they don’t move (or move very slowly)
b)
Pelagic
– these animals float in the water column
2.
Their
orientation to the environment isn’t of obvious importance
C.
These
animals usually have spherical or radial symmetry (Fig. 9-10)
1.
Spherical
symmetry means that the animal can be divided into mirror images regardless of
the orientation of the plane dividing the organism
a)
Found
chiefly in unicellular organisms – rare in multicellular animals
2.
Radial
symmetry occurs when mirror images are obtained when the animal is split into
two halves along one axis