PPT BY VEENA on cleavage types gastrula blastula morphogenetic movement s

1. S.Y.B.Sc ZOOLOGY SEMESTER
IV
COURSE X UNIT 1
COMPARATIVE EMBRYOLOGY
LECTURE :03

2. Meroblastic cleavage
• Incomplete divisions due to large amount of yolk present
• Telolecithal, centrolecithal (Macroleithal eggs)
• 2 Main types of Meroblastic Cleavage: Discoidal & Superficial
Discoidal cleavage
• Seen in animals having
discoidal egg (cytoplasm
less & yolk more)
• Cleavage is restricted to
only cytoplasmic disc
• Eg: Reptiles and birds
Superficial cleavage
• Limited to thin surface area of cytoplasm
• Cleavage doesn’t takes place in cytoplasm
but only nucleus divides (Kayokinesis)
• Divided nuclei are called Energids.
• These energids migrtae towards the
periphery of egg , syncytial blastoderm
• Cellularization occur at about 14th nuclear
division and create cellular blastoderm
• Eg : Insects (centrolecithal eggs)

3. DISCOIDAL CLEVAGE

5. TYPES OF BLASTULAE
• COELOBLASTULA
• DISCOBLASTULA
• BLASTOCYST
• Blastula : Cleavage leads to the formation of multicellular
embryonic stage called Blastula
• The cells of blastula are called Blastomeres
• Central cavity in blastula: Blastocoel/ segmentation cavity
(surrounded by blastoderm)

7. Coeloblastula
• Common in eggs having low yolk content
• Eccentric/centric blastocoel
• Blasstoderm : unilayered/multilayered
• Eg; Frogs, Amphioxus
• In some cases coleoblastula itself will play role of larva : Sponges
(Stereoblastula, Parenchymula, Amphiblastula directly develop into adult

10. DISCOBLASTULA
• Heavy yolk content : clevage occurs at Animal Pole hence leads to
the formation of polar blastomeres
• Blastodisc : a flat, topical multilayered disc of blastomeres
• Shows presence of subgerminal cavity or primary cavity

11. Blastocyst
• Marsupial & Placental Mammals (No/negligible amount of yolk)
• Cells at VP divide faster than cells at AP
• Cells at VP divide rapidly and constitute a thin outer wall called :
Trophoblast
• Shows presence of inner cell mass (confined to one pole)
• ICM is covered by layer of Rauber’s cell.
• ICM : Hypo & Epiblast

13. GASTRULATION
• Making & shaping of embryo by massive proliferation, movement &
rearrangements of cells occurred in simple multicellular blastula.
• The cell movements lead to formation of germ layers (embryonic layers:
ectoderm, endoderm & mesoderm) which play vital role in organogenesis
• Cell Movements (motility, shaping & adhesion)- Morphogenetic
movements
• Archenteron : new cavity developed during morphogenetic movements
• Archenteron opens to exterior by blastopore.
• 2 germ layers : Diplobalstic
• 3 germ layers: Triploblastic

17. MORPHOGENETIC MOVEMENTS
EPIBOLY
• Process of thinning and extension of
epithellium which occurs through
different movements
• Extension: Cells get stretched & increase
the germ layer length
• Intercalation: Merging of 2 epithelia in
which cells from 2 adjacent epithelia move
between one another and merge to form
single layer
• Convergence/divergence: Intercalation of
cells from multilayered epithellium in a
highly directional & Organized manner
• Cells Congrete at one point : Convergence
• Cells move away from each other:
Divergence
EMBOLY
• Movement of cell into interior
of embryo
• Invagination
• Ingression
• Infiltration
• Delamination
• Proliferation
• Involution

18. Types of morphogenetic movements