This document provides an overview of comparative embryology topics including types of eggs, sperms, cleavage, and blastula formation. It begins with classifications of eggs based on yolk amount and distribution. It then describes the structure and types of sperms in animals. The document outlines two main types of cleavage - holoblastic and meroblastic - and discusses equal and unequal holoblastic cleavage patterns. Finally, it introduces additional embryology concepts that will be covered such as gastrulation, coelom formation, and extraembryonic membranes.

1. S.Y.B.Sc ZOOLOGY
SEMESTERIV COURSE X
UNIT1 : COMPARATIVE EMBRYOLOGY

2. CONTENTS
1.1 Types of eggs
1.1.1. Based on amount of yolk
1.1.2 Distribution of yolk
1.1.3 Mosaic & regulative eggs
1.2 Structure & types of sperms
1.2.1 Basic structure of sperm
1.2.2 Types of sperms in animals
1.3 Types of cleavage: Holoblastic & Meroblastic
1.3.2 Meroblastic cleavage
1.4 Types of blastulae
1.5 Gastrulation : 1.5.1 – Morphogenetic movements
1.6 Coelom formation & types
1.6.1 Formation of coelom (schizocoely & enterocoely)
1.6.2 types of coelom
1.7 Extra embryonic membranes & types of placentae
1.7.1 Extra embryonic membrane
1.7.2 Types of placentae – Based on histology , Morphology and implantation
implantation

3. INTRODUCTION
Steps Involved During The Development Of An
Organism
• Fertilization
• Cleavage
• Blastulation
• Gastrulation
• Morphogenesis
• Coelom Formation
• Formation Of Extraembryonic Membranes

5. Schematic diagram showing animal and vegetal poles of egg

6. Classification of eggs of animals based on amount
and distribution of yolk
Based on amount of yolk
Oligolecithal
holoblastic cleavge Polylecithal (meroblastic cleavage)
Alecithal
• Negligible amount of yolk
• Embryo needs to be implanted
immediately after fertilization to
form placenta
• Direct development
• Eg; Eutherians (placental
mammals)
Microlecithal
Very little and evenly distributed yolk
Eg. Insects, tunicates
Indirect insect dec (metamorphosis)
Mesolecithal
• Moderate amount of yolk
confined largerly to
vegital pole
• Holobalstic cleavage
eg. Amphibians, petromyzon,
dipnoi
Macrolecithal
• Large amount of yolk
Eg. Birds, reptiles
• Complete embryo
growth inside the egg
befpre hatching

8. Based on distribution of yolk
Isolecithal/homolecithal egg
• Very little amount of yolk
• Distributed uniformly in
cytoplasm
• Holoblastic cleavage
• Eg. Echinoderms, mammals ,
amphioxus
Telolecithal
i) Slightly telolecithal
ii) Moderately telolecithal
iii) Pronounced telolecithal
Centrolecithal
Center of egg is
occupied by
nucleus & small
amt of cytoplasm
covered by thick
layer of yolk

9. Type of egg /term Amount of
yolk
Yolk distribution Cleavage Example
Alecithal Negligible/absent --- Holoblastic Eutherians
(Placental
mammals)
Microlecithal Very little Even Holoblastic Amphioxus
mesolecithal Moderate Uneven (vegital pole) Holobastic Petromyzon
Amphbians
Dipnoi
Macrolecithal Large Uneven (vegital pole) Meroblastic Birds,
Reptiles
Isolecithal/homolecithal Very little Uniform/even Holoblastic Echinoderms ,
mammals
Telolecithal Very high Ecentric/uneven (v.p) Meroblastic Birds
Slight: fishes
Moderate:
amphibian
Pronounced;
birds, reptiles
Centrolecithal Moderate Center , even Meroblastic Arthropods &
colelenterates

10. Mosaic and regulative eggs
Mosaic/Determinate eggs
• Every part to be developed of
the future embryo is
predetermined in egg itself
before /at time of fertilization
• If any portion is removed
from the egg then related
organ will not developed
• Autonomous specification
Eg: polychaetes, Nimertine (Ribbon
terms), Annelids, Molluscs and
Ascidians
Regulative/Inderminate egg
• Eggs in which developmental
potentialities for different organs
of future embryo are not
predetermined
• Determination occurs only after
third cleavage
• Conditional Specification
• If blastomere of such an egg is
removed before third cleavgae, it
develops into complete embryo
eg. Human identical twins
development
Cleidoic egg
Eggs with calcareous shell
(dry lands)
Internal development
Eg. Birds & reptiles
Non-Cledoic egg
Eggs without shell
Eg. Mammals

12. FILL UP THE ATTENDANCE FORM TIME LIMIT IS 5.10

13. LECTURE 02
03/12/2021
• TYPES OF SPERMS
• HOLOBLASTIC CLEAVAGE
• (EQUALAND UNEQUAL HOLOBLASTIC CLEAVAGE)
• CLEAVAGE TYPES BASED ON SYMMETRY

14. • O.018mm : Amphioxus
• 2.25mm in toad
• Shape of head varies from species to species
• Branched/star shaped in crusteceans and
salamanders
• Spherical in teleosts
• Lance shaped In amphibians and domestic
fowls
• Hook shaped in mouse and rat
• Spoon shaped : mammals
• Non-flagellate : Ascaris & some crusteceans
• Flagellated
• Uniflagellate: Human Male
• Biflagellate : Toad, fish & flatworm
• Multiflagellate :cladocerans/crusteceans
TYPES OF SPERM
Sperm of amphioxus

15. Sperm of teleost

16. CLEAVAGE IN EGGS
• Fertilized egg is a single cell structure which gives rise to multi-
cellular embryo thorough process of cleavage
• During Cleavage , zygote divides repeatedly by mitosis into
smaller cells called blastomeres
• Cleavage is characterized by rapid and sucessive mitotic
divisions without any intervening periods of growth.
• Blastomeres do not increase in size and the resulting blastomeres
are only half the original size
• The cleavage begins with one very large cell and ends with large
number of smaller cells

17. • Initial division Synchronous
• Blastomeres present divide and form solid ball of cells:
Morula
• Synchrony disappears in later stages
• Formation of blastocoel ( cavity)
• Resting multicellular structure (embryo) formed as a result
of cleavage called blastula

19. EQUAL HOLOBLASTIC CLEAVAGE
• Micro and Isolecithal eggs
• Egg divides and produces cells of roughly same size
• First and second mitotic divisions are meridonial and right angle to each
other
• Plane of third division is horizontal to equator
• Eg. Marsupials, placental mammals, amphioxus

20. • Unequal holoblastic cleavage
• Meso and telolecithal eggs
• Micromeres and macromeres/megameres ; Small and Large sized blastomeres
• Eg. Bony fish and amphibians
• Moderate amount of yolk is present towards vegital pole & it retards cleavage
furrows in in the vegital pole
• First n second div are meridonial and right angle to each other
• Plane of third cleavage is horizontal but above the equator
• Blastomeres of unequal size are formed

23. • Bilateral cleavage
• Unequal holoblastic
• First; right and left half : bisects zygote longitudinally 9mirror images of each
other)
• Blastomeres are arrranged in bilaterlly symmetrical manner
• eg. Vertebrates, molluscs and echinoderms
• Radial Cleavage
• Equal Holoblastic division
• Cell divisions are equal symmetrical and occur right angle to the polar axis of
egg
• Blastula is divisible into 2 mirror images along axis drawn on any radius i.e
Radially symmetrical
• Eg. Echinoderms and amphioxus
• Spiral cleavage
• Cells are placed alternate to each other /spiral in fashion
• Equal holoblastic cleavage
• Eg. Flatworms, Molluscs , annelids

24. Example: Annelids, Flatworm, Molluscs

25. ROTATIONAL CLEAVAGE
EXAMPLE: MAMMALIAN EGGS

26. Equal Holoblastic cleavage
• In microlecithal or isolecithal eggs
• Egg divides and forms blastomeres
of same size
• 1st & 2nd divisions are meridonial
& right angle to each other
• 3rd cleavage is at equator , as a
result of which 8 blastomeres of
equal size are formed
• Eg. Amphioxus, Marsupial &
Placental mammals
Unequal Holoblastic
Cleavage
• In Meso & Telolecithal eggs
• Produces blastomeres of
unqual size
• Small sized (Micromeres) &
Large sized
(Megameres/macromeres)
• 1st and 2nd cleavage are
meridonial & right angle to
each other, 3rd cleavage is
above the equator which
results in formation of
unequal sized blastomeres
• Eg. Bony fishes and
Amphibians

27. HOLOBLASTIC EQUAL CLEAVAGE HOLOBLASTIC UNEQUAL CLEAVAGE