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Animal Development
Bio 1413: General Zoology Laboratory
Ziser, 2008
All living organisms exhibit some form of growth and development. Members of the animal kingdom
have the most complex developmental cycle of any living organism. The sequence of discrete,
recognizable stages that these organism pass through as they develop from the formation of a zygote (the
fertilized egg) to the sexually mature adult are referred to as its developmental cycle.Animal
development can be subdivided into several sequential processes: gametogenesis, fertilization,
embryonic development and post embryonic development. Embryonic development includes the
processes of growth, determination, differentiation and morphogenesis.
1. Gametes. The gametes are produced by the process of meiosis which differs from mitosis in that
only one of each chromosome ends up in the cells after division. The male gamete, the sperm, is small
and almost always flagellated. The female gamete us usually large since it contains yolk, and spherical.
slides:
sperm smear
starfish unfertilized eggs
Activity Be able to distinguish between sperm and eggs and to find the following structures on
slides and illustrations:
for sperm identify: head, middle piece, tail (flagellum)
for egg identify: cell membrane, nucleus, nucleolus
2. Fertilization. At fertilization only a single sperm penetrates and adds its chromosomes to those in
the egg. The fertilized egg then has a pair of each chromosomes, one each from the male parent and the
other of each from the female parent. To prevent additional sperm from penetrating the egg a fertilization
cone is produced to produce the original sperm into the egg quickly. Then a fertilization membrane
expands around the egg and pushes away and “locks out” other sperm cells.
Slide:
starfish early cleavage, Asterias eggs
Activity Be able to distinguish between unfertilized and fertilized eggs
3. Embryonic Development - identify the various stages of starfish embryology
as illustrated in the lab manual and as seen on the following slides.
A. Cleavage. The first identifiable period of embryonic development occurs as the fertilized egg
begins to divide. These early divisions are called cleavage divisions and each cell produced is a
blastomere.
Slide: starfish –early cleavage
Activity observe the slide of starfish eggs in early cleavage and be able to distinguish this
stage from those that follow
-note 2,4,8 cell stages
-blastomeres
B. Morula: continued division leads to a solid ball of cells called the morula.
Slide: starfish, late cleavage
Activity identify the morula stage. How does its size compare with that of the fertilized
egg?
C. Blastula or Blastocyst: If the embryo is spherical it is called a blastula; if it is a flattened
disc it is called a blastocyst, both are equivalent stages. At the blastula stage, the cells form a
hollow sphere. The cavity inside is called the blastocoel.
Slide: starfish blastula; starfish development
Activity Identify the blastula and distinguish it from other embryological stages
-hollow ball of cells
-blastocoel
D. Gastrula or Primitive Streak:
In the gastrula, a depression forms at one end of the embryo, cells move in to form a saclike
pouch. The embryo is now essentially two layered. The cavity of this new pouch is called
the archenteron which is surrounded by the original, now much smaller blastocoel. The
archenteron opens to the outside through the blastopore. The blastopore will eventually become
either the mouth or the anus of the adult animal.
Slide: Starfish gastrula, starfish development
Activity Identify the blastula and distinguish it from other embryological stages. Note the
structures below
-invagination
-primary germ layers: ectoderm, endoderm
-archenteron
-blastopore
In the case of the blastocyst, the flattened embryo, the next stage is called the primitive streak
stage. A slit forms in the center of the sheet of cells (= the primitive streak) and cells migrate into
the slit and under the original layer of cells to form a double layered disc. This stage is equivalent
to the gastrula above.
Slide: chick, 18 hr, wm
Activity Identify the primitive streak stage and understand what is occurring here. Note
the following structures:
-original blastocyst
-primitive streak
-double cell layer; primary germ layers: ectoderm and endoderm
4. Post Embryonic Development. In animals the embryo may develop into any of several forms, some
that resemble the adult, some that live and feed independently and others that are completely dependent on
the mother for sustenance.
A. Larvae. Further development varies considerably in different animal groups. In some animals,
especially marine ones, the embryo soon develops into a freeliving larval form. While each
phylum and sometimes each class usually has its own characteristic larvae, there are a few larval
forms that are found in more than one phylum. Similar larvae imply similar ancestry; indicating
that the phyla are relatively closely related. The larvae listed below are evolutionarily significant.
You should be able to distinguish them from each other and name them. (You do not need to
know to which group each belongs…yet)
Slides:
Aurelia planula, wm
Patella trochophore larvae, wm
Nauplius, barnacle, wm
Starfish bipinnaria larva, wm
1. Planula. A simple multicellular, oval larva with no discernable organs its surface is
covered with cilia
2. Trochophore. A top-shaped larva with a digestive tract beginning at the mouth and
terminating in an anus. Tufts of cilia are found at each end and bands of cilia surround
the wider central area of the larva
*Note: the trochophore slides we have are not the best and very few of the larvae on the slide are oriented
properly to show the above characteristics.
3. Nauplius. A triangular larva with three pairs of jointed appendages, eyespots, and
digestive organs.
4. Bipinnaria. An elongated larva that bulges at both ends, with a curving digestive tract
with mouth and anus, and bands of cilia encircling the larva
B. Nymphs: Nymphs are immature stages of animals that at least somewhat resemble
the adult of the species and that live and feed independently
preserved:
mayfly, dragonfly, stonefly nymphs
C. Fetus: In addition to embryonic development, vertebrates (higher animals) produce an
immature stage that does resemble the adult but that is usually completely dependent on
the mother for nutrition and protection.
preserved:
illustrations:
misc vertebrate fetuses: human, shark, horse, etc
human fetal development