1) Cleavage

Embryonic development The embryonic development of any animal starts from the fertilized egg (zygote) which usually passes through three main stages which are: 1) Cleavage, 2) Gastrulation and 3) Organ formation (organogenesis).

1) Cleavage: After fertilization, the zygote divides repeatedly by a series of mitotic divisions. Zygote

vertical

2-blastomere stage

at right angle to the horizontal 4-blastomere stage 1st division double vertical 8-blastomere stage 16-blastomere stage double horizontal 32-blastomere stage (morula) 64-blastomere stage

A blastula

128-blastomere stage

The blastula @ It is a hollow structure formed at the end of cleavage process. @ Its wall is consisted of a single layer of cells. These cells are differentiated into micromeres at the animal pole and macromeres at the vegetal pole. @ The fluid filled cavity in its center is termed blastocoel. This blastocoel is not connected to the exterior.

2) Gastrulation

The gastrula @ It is an elongated structure formed at the end of gastrulation process by flattening and invagination of macromeres of blastula. Invagination continues until the macromeres come in direct contact with micromeres. So, the blastocoel is disappeared while a new cavity is formed. @ Its wall is formed from a double layers of cells. The outer layer which is formed from micromeres (is known as the ectoderm) while the inner layer is formed from the macromeres (the endoderm forms). @ It has a cavity that called archenteron which is connected to the exterior through an opening called a blastopore.

3) Organ formation (organogenesis): Zygote

Blastula Gastrula Ectoderm Epidermis Brain & nervous and associated system structures e.g. skin, hair, nails … etc.

Endoderm

Mesoderm Notochord

Somites

Mesenchyme (loose & migrating Muscles cells)

Dermis Circulating Bones & (inner system cartilage skin layer (heart & blood Excretory vessels) system

Embrygonic gut

Outer Inner Inner lining covering of lining of of the respiratory internal digestive tract tract organs

Gonads

Glands including liver & pancreas

Histology Types of tissues Epithelial tissues Origin

Ectoderm, endoderm or mesoderm

Small Matrix (intercellu- amount lar substance) Basement Has a membrane basement membrane

Connective tissue mesoderm

Huge amount

Has not basement membrane

Muscular tissues

Nervous tissues

Mesoderm,

Ectoderm,

Epithelial tissues According to their function

Covering epithelium

Glandular epithelium

Neuro-epithelium

According to the number of layers of cells

Simple epithelium Pseudostratified epithelium Stratified epith.elium (one layer) (all cells are not attached to the (more than one layer) free surface According to the shape of cells & the presence or absence of cilia on Lining blood vessels the free surface 1- Simple squamous epithelium Thyroid glands 2- Simple cuboidal epithelium 3- Simple columnar epithelium 4- Simple columnar ciliated epithelium

Lining digestive tract of Bufo

Lining ovisac & oviduct of Bufo

Epithelial tissues According to their function

Covering epithelium

Glandular epithelium

Neuro-epithelium

According to the number of layers of cells

Simple epithelium

Pseudostratified epithelium

According to the presence or absence of cilia on the free surface 1- Pseudostratified columnar epithelium e.g. male urethra

2- Pseudostratified ciliated columnar epithelium e.g. lining trachea

Stratified epith.elium

Covering epithelium Simple epithelium

Glandular epithelium

Neuro-epithelium

Pseudostratified Stratified epithelium epith.elium According to the shape of the cells & the presence or absence of cilia on the free surface

1- Stratified squamous epithelium e.g. Epidermis of the skin 2- Stratified cuboidal epithelium e.g. Cloaca of the toad 3- Stratified columnar epithelium e.g. Conjunctiva of the eye 4- Stratified ciliated columnar epithelium e.g. lining vas deferens 5 Transional epithelium e.g. lining urinary bladder

Covering epithelium Endocrine glands -without ducts)(ductluss glands)

Glandular epithelium

Neuro-epithelium

According to the presence or absence of ducts

Unicellular e.g. pituitary, thyroid, glands parathyroid, adrenal, -One cellgonads (testes & e.g. goblet cells ovaries), islets of Langerhans, According to the shape Simple glands (have one of secretory portion duct) 1S. tubular glands e.g. crypts of Lieberkühn 2- S. coiled tubular glands e.g. sweat glands 3- S. branched tubular glands e.g. gastric glands 4- S. alveolar glands e.g. mucous & poisonous gland in skin of Bufo 5- S. branched alveolar glands e.g. sebaccous glands

Exocrine glands -With ducts(duct glands)

According to the number of cells Multicellular glands - Moretothan cell According the one duct system Compound glands (have duct system)

Covering epithelium

Glandular epithelium

Endocrine glands (ductluss glands)

Neuro-epithelium

Exocrine glands (duct glands) According to the number of cells

Unicellular e.g. pituitary, thyroid, glands e.g. goblet cells parathyroid, adrenal, gonads (testes & ovaries), islets of Simple glands Langerhans, …. Etc.

Multicellular glands

Compound glands According to the shape of secretory portion 1C. tubular glands e.g. lacrimal glands 2C. alveolar glands e.g. salivary gland 3C. tubuloalveolar glands e.g. mammary gland & exocrine portion of pancreas 4- Reticular glands e.g. liver of mammals

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