Bases of Animal Classification

1. BASES OF ANIMAL
CLASSIFICATION
MD NASIF SADAT, LECTURER (Zoology), BCS (General Education)
Department of Zoology
NARSINGDI GOVERNMENT COLLEGE

2. TOPICS
Embryology
Germinal Layers
Cleavage and its Types
Fate of germinal layers
Protostome and Duterostome

3. CLEAVAGE
 These cleavage-stage cells are called blastomeres.
 During cleavage, however, cytoplasmic volume does
not increase. Rather, the enormous volume of zygote
cytoplasm is divided into increasingly smaller cells.
 One consequence of this rapid cell division is that the
ratio of cytoplasmic to nuclear volume gets
increasingly smaller as cleavage progresses.
 This decrease in the cytoplasmic to nuclear volume
ratio is crucial in timing the activation of certain
genes. For example, in the frog Xenopus laevis,
transcription of new messages is not activated until
after 12 divisions. At that time, the rate of cleavage
decreases, the blastomeres become motile, and
nuclear genes begin to be transcribed. This stage is
called the midblastula transition.
 Thus, cleavage begins soon after fertilization and ends
shortly after the stage when the embryo achieves a
new balance between nucleus and cytoplasm.
Cleavage, a series of mitotic divisions whereby the enormous volume of egg cytoplasm is
divided into numerous smaller, nucleated cells.

4. After fertilization the zygote forms a solid mass of cells by mitotic division. This
multicellular solid structure is called morula.
Fertilization leads to the formation of a zygote. During the next stage, cleavage, mitotic cell
divisions transform the zygote into a hollow ball of cells, a blastula.
This process of forming blastula after fertilization is called blastulation.
This early embryonic form, blastula, undergoes further development, forming a gastrula with
either two or three layers (the germ layers). In all vertebrates, these progenitor cells differentiate
into all adult tissues and organs.
This process of forming gastrula is termed as gastrulation.
CLEAVAGE

7. GERM LAYERS

8. GERM LAYERS
Fate of Ectoderm
(1) Epidermis of skin, hair, arrector pili muscles, nails, sudoriferous (sweat) and sebaceous (oil)
glands and chromatophores (pigment cells) of skin.
(2) Enamel of teeth, salivary glands, mucous membrane of lips, cheeks, gums, part of the floor of
the mouth and part of palate, nasal cavities and paranasal sinuses. Lower part of anal canal.
(3) Terminal part of male urethra.
(4) Nervous system including all neurons, neuroglia (except microglia), and Schwann cells.
Piamater and arachnoid mater.
(5) External ear, outer layer of tympanic membrane, membranous labyrinth (internal ear).
(6) Mammary glands, outer surface of labia minora and whole of labia majora.
(7) Pituitary gland, pineal gland and medulla of adrenal glands.
(8) Conjunctiva, cornea, lens of eye, muscles of iris, vitreous humour, retina, lacrimal gland.

9. Fate of mesoderm
(1) Muscles except iris muscles.
(2) Connective tissues including loose areolar tissue, ligaments, tendons and the dermis
of skin.
(3) Specialized connective tissues like adipose tissue, reticular tissue, cartilage and
bone.
(4) Basis of tympanic membrane.
(5) Coelomic epithelium (mesothelium of pleural, pericardial and peritoneal cavities).
(6) Kidneys, ureters, trigone of urinary bladder.
(7) Heart, all blood vessels, lymphatics, blood cells, spleen.
(8) Duramater, microglia.
(9) Sclera, choroid, ciliary body and iris.
(10) Cortex of adrenal glands.
(11) Dentine of teeth.
(12) Mesenteries.
(13) Notochord.
(14) Reproductive system except prostate.
GERM LAYERS

10. Fate of endoderm
(1) Epithelium of mouth, part of palate, tongue, tonsils, pharynx,
oesophagus, stomach, small and large intestines including upper part of
anal canal (not lower part of anal canal).
(2) Epithelium of Eustachian tube, middle ear, inner layer of tympanic
membrane.
(3) Epithelium of gall bladder, liver, pancreas including islets of Langerhans,
gastric and intestinal glands.
(4) Epithelium of larynx, trachea, bronchi and lungs.
(5) Epithelium of urinary bladder except trigone.
(6) Epithelium of lower part of vagina, vestibule and inner surface of labia
minora.
(7) Epithelium of thyroid, parathyroid and thymus glands.
(8) Epithelium of prostate (except inner glandular zone), bulbourethral
GERM LAYERS

11. TYPES OF CLEAVAGE
Based on yolk of an egg –
1. Holoblastic Cleavage
2. Meroblastic Cleavage
Based on Symmetry –
1. Radial Cleavage
2. Spiral Cleavage
Based on fate of Blastomere –
1. Determinate Cleavage
2. Indeterminate Cleavage

12. TYPES OF CLEAVAGE
a. Holoblastic or total cleavage: When the
cleavage furrows divide the entire egg. It may
be:
i) Equal: When the cleavage furrow cuts the
egg into two equal cells. It may be radially
symmetrical, bilaterally, symmetrical, spirally
symmetrical or irregular.
ii) Unequal: When the resultant blastomeres
become unequal in size.

13. TYPES OF CLEAVAGE
b. Meroblastic cleavage: When segmentation
takes place only in a small portion of the egg
resulting in the formation of blastoderm, it is
called meroblastic cleavage. Usually the
blastoderm is present in the animal pole and the
vegetal pole becomes laden with yolk which
remains in an uncleaved state, i.e., the plane of
division does not reach the periphery of
blastoderm or blastodisc.
c. Transitional cleavage: In many eggs, the
cleavage is atypical which is neither typically
holoblastic nor meroblastic, but assumes a
transitional stage between the two.

14. • Radial – the cells divide such that each
cell in the top four cell plane is directly
over one other cell in the bottom plane.
Example: Microlecithal and isolecithal
eggs.
• Spiral - the cells divide at slight angles
to one another, so that the none of the
four cells in one plane of the eight-cell
stage is directly over a cell in the other
plane
TYPES OF CLEAVAGE

15. Determinate: The developmental fate of each embryonic cell is
established very early. If a cell is isolated from the 4-cell stage the
embryo will not fully develop. This is because the fate of each
blastomere is predetermined in the early embryonic stage itself.
Annelids, mollusks and ascidians which produce mosaic type of eggs
exhibit determinate cleavage.
Indeterminate: Early embryonic cells retain capacity to develop
into a complete embryo if isolated from other cells. Cleavage
produces blastomeres which are qualitatively equipotential or
totipotent. When they are isolated, they develop into complete
embryos. This is because the fates of blastomeres are not
predetermined in the early embryonic period. Vertebrates and
certain invertebrates such as echinoderms which produce regulative
type of eggs exhibit indeterminate cleavage.
TYPES OF CLEAVAGE

16. PROTOSTOMIA Vs DUTEROSTOMIA
The main difference between protostomes and
deuterostomes is that the blastopore in
protostomes are developed into a mouth while
the blastopore in deuterostomes is developed
into an anal opening.
Greek words “proto” meaning first, and “stoma”, which means opening
or mouth. In literal terms, protostome means ‘first mouth’.
Greek origin; “deutero” means second, and “stoma” means mouth or
opening. Thus, deuterostome translates to ‘second mouth’. In
deuterostomes, the opening at the bottom of the gastrula, known as
the blastopore, forms the anus.

17. Comparison Between Protostomes and Deuterostomes
Now let’s go through some other striking differences between protostomes and deuterostomes:
1. Origin of Mouth and Anus: In protostomes, the origin of the anus is secondary, but in deuterostomes, the
formation of the mouth is secondary.
2. Body Complexity: Deuterostomes have a more evolved and complex body organisation than protostomes.
3. Type of Cell Ciliation: The cells in protostomes are multi-ciliated, but those in deuterostomes are mono-
ciliated.
4. Number of Phyla: Protostomes include more species and phyla compared to deuterostomes.
5. Classification: Protostomes can be grouped into Spiralia and Ecdysozoa. The groups under deuterostomia
include Echinoderms, Chordates, Hemichordates, and other higher and more complex organisms like human
beings.
PROTOSTOMIA Vs DUTEROSTOMIA

18. 6. Origin of Mesoderm: The archenteron is the rudimentary alimentary canal formed during the early
stages of embryonic development that later forms the mesoderm and endoderm. The development of
archenteron is not seen in protostomes, but, for deuterostomes, the primitive gut formation takes place
in early embryos.
7. Late Gut Development: In protostomes, the anus is formed by tunnelling of the gut into the embryo.
But in deuterostomes, the formation of the mouth is by the gut tunnelling into the embryo.
8. Development of Coelom: In protostomes, the body cavity of the coelom is formed by splitting the
mesodermal layer. Thus, they are schizocoelomates since the coelom is created by schizocoely. On the
other hand, the deuterostomes are enterocoelomates where the body cavity or the coelom is formed by
the mesoderm pinching off from the gut.
9. Type of Cleavage: In protostomes, the kind of cleavage is determinate, meaning that the blastomere
of the early embryonic stage is incapable of developing into independent embryos. But deuterostomes
undergo indeterminate division whereby the early blastomeres can develop into complete embryos.
10. Nervous System: Protostomes have a solid, ventral nerve cord. In deuterostomes, the nerve cord is
hollow, with some possessing pharyngeal gill slits.

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