Characteristics of cleavage divisions

1) All cleavage divisions are mitotic in nature and occur rapidly one after another, the rate depends on the species. The first cleavage in human zygote occurs in about 24 hours, while in a frog’s egg may occur within 90 min.

2) The resulting daughter cells formed are called as blastomeres.

3) The divisions are so rapid that there is no time for blastomeres to grow and hence the blastomeres become progressively smaller.

4) Since zona pellucida is intact, the size of morula is almost same as that of the fertilized ovum.

5) As more and more blastomeres are formed

F irst cleavage is vertical and passes through the animal – vegetal axis dividing the zygote into 2 – two blastomeres.

S econd cleavage is again vertical and at right angles to the first one and divides both the blastomeres resulting in a four cell stage. Second cleavage occurs about 60 hours after fertilization.

T hird cleavage takes place about 72 hours after fertilization. It is a horizontal division at right angles to the first two, dividing all the four blastomeres horizontally, forming eight cell stage.

Successive cleavage division occur one after another in a highly organized and orderly manner producing a spherical mass of a cells called the morulla (meaning little mulberry).

Morulla consist of 16-32 cells but is the same size as the zygote.

Types of gastrulation

1) Immigration – separate cells of blastoderm immigrated from surface of blastula inside and to form 2-layered embryo outer layer becomes ectoderm and inner layer - endoderm.

2) Invagination – vegetative pole of blastula screw in blastocoel and reach animal pole. (isolecithal eggs with complete uniform cleavage).

3) Epiboly – if embryo developed from telolecithal egg and on blastula’s vegetative pole have large amount of yolk and vegetative pole macromeres can not invaginated to blastocoel. And gastrulation occurs by fast division of animal pole’s micromeres, which are become, overgrown cells of vegetative pole.

4) Delamination (segregation or splitting) in coelenterates. Each cell of blastoderm is divided into outer and inner and to form ectoderm and endoderm.

Primary germ layers

Ectoderm forms the outermost layer of the embryo and gives rise to following structures:

1) skin - epidermis and epidermal glands of skin like sweat, sebaceous, etc., sense receptors of epidermis

2) brain, spinal cord, entire nervous system

3) enamel of teeth, hair and nails

4) mucous membrane of lips, cheeks, gums, floor of the mouth, part of the palate and nasal cavities

5) conjuctiva, lens, cornea and retina of eyes

6) adrenal medulla, outer layer of tympanic membrane

7) mammary and lacrimal glands.

Endoderm is the innermost layer of embryo gives rise:

1) to lining of alimentary canal, mucous epithelium of gut, epithelium of pharingo-tympanic tube, middle ear and inner layer of tympanic membrane.

2) respiratory system, liver, pancreas, glands of gastrointestinal tract.

3) thyroid, parathyroid, thymus, islets of Langerhans.

4) urinary bladder, urethra, lower part of vagina, vestibule.

Mesoderm is the middle layer gives rise to

1) skeleton, muscular system.

2) circulatory system, heart, blood vessels, lymph system.

3) kidneys, ureters.

4) reproductive system: ovary, uterus, uterine tubes in females and testes, epididymus, vas defferens, seminal vesicles and ejaculatory ducts.

5) connective tissue, dermis of skin, dentine of teeth.

6) lining of pleural, pericardial and peritoneal cavities, iris, sclerotic and choroid of eyes.

7) adrenal cortex.