Embryogenesis (development of embryo) includes next stages: a) FERTILISATION (fusing of
 male  and female gamets, resulting in ZYGOT), b) CLEAVAGE – mitotic division of zygote, resulting in BLASTULA formation, c) GASTRULATION – differentiation of blastula wall on 3 germ layers (ectoderm, mesoderm, endoderm) and 3 axial organs (notochord, neural tube, gut), d) HISTOGENESIS -  differentiation of germ layers in tissues, e) ORGANOGENESIS – differentiation of tissues in organs.
OOCYTE   – is  immature female sex cell. OVUM –is MATURE gamet. Both  contain YOLK inclusions  in cytoplasm.(yolk= lecithos).     Classification  of oocytes/ovums  according yolk AMOUNT:
   a) OLIGOlecithal oocyte –it contains SMALL amount of yolk (in classes : 1.mammals, including humans, 2.in primitive chordate), b) MESOlecithal oocytes- it contains moderate amount of yolk 
  (in class amphibia),    c) POLYlecithal ovum-  it contains big amount of yolk ( in class birds).  a) ISOlecithal oocytes – yolk droplets are distributed equally, NO poles, b)TELOlecithal ovums are POLAR gamets : yolk locates on vegetative pole, but nucleus and organells are on animal pole. Telolecithal ovums may be:  - -  SHARPLY telolecithal and   - -  MODERATELY  telolecithal.
   Mammalin/HUMAN ovum is  :secondary OLIGOlecithal ISOlecithal.    Amphibian ovum is: MESOlecithal, moderately TELOlecithal.   Bird ovum is : POLYlecithal, sharply TELOlecithal.     
         Mammalian/HUMAN ovum has 3 tunics and contains in ooplasm 2 types of granules:  
           1) YOLK granules,       2) CORTICAL granules (it contain GAGs and lytic enzymes).    
                                      3 TUNICS,  surrounding HUMAN ovum: 
   1) OOLEMMA (plasma membrane),      2) ZONA PELLUCIDA (contains:GAGs, including 
                                   hyaluronic acid,     and  glycoproteins  ZP1-, ZP2-, ZP3-fractions). 
      3) CORONA RADIATA.  It is composed of cells of follicular epithelium..
          Abbrev. “GAGs”  means:  “GlycosAminoGlycans”  (groop of compound carbohydrates).
         FERTILIZATION in mammalian/human happens in upper 1/3 of fallopian tube and includes 
 3 phases: 1) DISTANT interaction. During this CAPACITATION happens – it is removal of mask 
  proteins (glycocalyx) from sperm head and sperm becomes is able to recognize ZPs of zona pellucida.
       2) CONTACT interaction. It includes :ACROSOME reaction – acrosome releases lytic enzymes, destroying 3 ovum tunics. Sperm head enters in ooplasm and initiates CORTICAL reaction – cortical granules release it contant, resulting in transformation of  zona pellucida into  nonpermeable FERTILISATION MEMBRANE.  It prevents polyspermia (entrance of more than 1 spermatozoon).
    3) INFILTRATION.  Nuclei of sperm and ovum are fused, diploid set is restored, resulting in 
       ZYGOT formation.
   Zygot divides by mitosis (CLEAVAGE stage), resulting in BLASTULA formation. BLASTULA is vesicle-shaped multicellular embryo with:1) cavity, named BLASTOCELE, containing embryonic
 fluid, 2) wall, named BLASTODERM, consisting of embryonic cells, named BLASTOMERS.      1) COELOBLASTULA  (in primitive chordate).  Blastomers locate in 1 layer and are equal in sizes.
2) AMPHIBLASTULA (in amphibian). Animal pole contains small-sized microblastomers,  vegetative pole contains big-sized macroblastomers.  Both poles are multi-layered.
3) DISCOBLASTULA (in birds). Microblastomers of animal pole associate into disc, floating on the surface of yolk-containing  vegetative pole.  Blastocele is very narrow.
4) BLASTOCYST (in mammalian/HUMAN).   Blastoderm is composed 2 cellular masses:  
        a)inner cell mass, or EMBRYOBLAST,    and  b) outer cell mass, or TROPHOBLAST.  1) INVAGINATION  (in primitive chordate),      2) EPIBOLY (in amphibian), 
 3) DELAMINATION (it is early gastrulation in birds and in mammalian/HUMAN). Result of it : embryo becomes BILAMINAR  and composed of 2 germ layers : a)ectoderm and  b)endoderm. 4)IMMIGRATION (late gastrulation   in birds and in mammalian/HUMAN). 2 results of immigration: 
 1^st - embryo becomes TRILAMINAR  and is composed of 3 germ layers : a) ectoderm +  b) endoderm +c) MESODERM. 2^nd - 3 AXIAL organs are formed: a) NOTOCHORD, 
                                    b) NEURAL TUBE (from ECTOderm),    
                                    c) GUT, or primitive alimentary canal, ( from embryonic ENDOderm)