Involves:

• Rupture of one or more mature ovarian follicles
• Shedding of some of its contents from the surface of the ovary
• Remainder of the follicle is converted under hormonal influence into the corpus luteum

• Important endocrine structure
• Secretes progesterone & estrogens
  • These produce uterine changes that facilitate implantation & maintenance of the conceptus

1. Germ cell
2. 1st polar body
3. Zona pellucida
4. Corona radiate

• Usually a secondary oocyte, arrested in metaphase
• In horses and dogs, its a primary oocyte that has not completed division

1. Ovulated structures shed from the ovary surface to the peritoneal cavity
2. Pass into the oviduct
   • Assisted by the action of the cilia of the infundibulum
   • Second factor is the constant stream of peritoneal fluid entering the oviduct
3. Germ cell & related structures are assisted in passage to the uterus by muscular contractions of the oviduct & its cilia

1. Infundibulum
2. Ampulla
3. Isthmus

• The ability of sperm to fertilize the female germ cell
• Occurs within the female tract
• Involves biochemical changes within the sperm

• Partly by muscluar contraction of the female tract
• Partly by the motility of the sperm
• Cilia of the uterine tube do not assist because they beat towards the uterus

• Most species 1-2 days
• Horse & dog 6-7 days
• Birds 32 days in fowl; 70 days in turkey

1. Corona radiata
2. Zona pellucida
3. Cell membrane of oocyte

By the time fertilization is occuring, the corona radiate cells are already fragmenting

(in ox they have already disappeared completely)

• Tail of the sperm is shed
• Cell membranes of the female & male germs cells rupture & then unite together
• Thus, enclosure of sperm within the cytoplasm of the female cell

1. Completion of 2nd meiotic division of the oocyte
2. Block to polysperray through cortical reaction
3. Haploid female & male germ cells swell &  occupy a central position w/n cytoplasm to become pronuclei
4. Pronuclei fuse together to create a diploid zygote

• Prevents further sperm from penetrating the female germ cell
• Cortical granules in the oocyte cytoplasm align at the periphery of the ooctye, adjacent to the plasma membrane (cortical recation)

1. Prior to fusion, male & female pronuclei duplicate their DNA
2. Immediately on fusion, their chromosomes split in the course of normale mitotic division
3. This results in a 2-cell zygote

• Mammals: males
• Birds: females

• When more than one sperm penetrates the female germ cell
• 3 or more pronuclei may occur & fuse to make a triploid embryo
• Common in pigs
• Most die in early development, but rare viability in humans & cats

• After 2-cell stage, cell division occurs without increasing the conceptus size (still enveloped in the zona  pellucida)
• Through this cleavage or segmentation, the resulting daughter cells (called blastomeres) are progressively smaller and smaller
• Once the concetus is a solid ball of blastomeres, its called a morula ("little mulberry")

• Zona pellucida
• Frees the conceptus to enlarge & rapidly grow to fill the uterine lumen

• In the blastocyst, intercellular fluid-filled spaces start to appear
• These combine into 1 single, enclosed cavity called the blastocoele

1. Embryoblast
   • inner cell mass
2. Trophoblast
   • outter cell mass

• Most species: At end of th 1st week of development
• Dogs: At end of the 2nd week

• When the conceptus becomes attached to or embedded in position in the uterus
• Establishes the close link between foetal & maternal tissue necessary for development of the placenta

• Occurs in domestic species
• Embryo remains located in the uterine lumen

• In humans & guinea pigs
• Conceptus migrates through the uterine epithelium into the substance of the endometrium

• Changes in the endometrium from the action of progesterone (from the corpus luteum)
• Changes in the conceptus
  • Elongation of the blastocyst occurs (particuarly in the pig) to greatly increase the surface area of contact b/w the trophoblast & uterine epithelium
  • Simple adhesions develop
  • Junctional complexes form

• Pig: 12 days
• Dog: 18 days
• Sheep: 10 days & cmopleted 4-5 weeks later
• Horse: 2 months & completed ~ the 14th week

• Evenly along both horns
• Some transuterine migration may occur

• Dizygous
• Monozygous

• 2 separate ova are fertilized
• Genetically similar like normal brother/sister relationships

• 1 ovum is fertilized but divides at the blastocyst stage into 2 individuals
• Genetically identical
• Same sex & blood groups
• Notable in huamsn & cattle

• A fertilized ovum may be lost into the peritoneal cavity & become implanted into a peritoneal surface
• Usually results in early embryonic death

• Zona pellucida
• Avoids direct contact b/w trophoblast & maternal epithelium
• Delays expansion of the conceptus while still in the oviduct

• In dizygous twinning in cattle, the development of a common circulation by placental anastomosis usually occurs
• If the twins are opposite sex, the female develops a type of intersex known as freemartin

1. Ectoderm
2. Mesoderm
3. Endoderm

• Delamination
• The embryoblast mass cells give rise to a second inner layer or smaller, darker cells (the embryonic endoderm)

• Covering of trophoblast cells over the embryoblast disappears
• Exposed cells of the embryoblast proliferate to form the epiblast

• Produces the bilaminar germ disc/embryonic disc that is continuous with the trophoblast

• Endoderm spreads outwards from the germ disc to line the internal aspect of the trophoblast that is now called the trophectoderm
• The endoderm so formed is called the hypoblast and encloses the yolk sac or primitive gut
• 2 layered wall is called the bilaminar omphalopleure

• Rapid growth of the embryonic disc occurs in an uneven manner, converting the ovoid disc into a pear-shaped structure
• A whitish line, called the primitive streak, appears on the embryonic ectodermal layer
• This establishes the polarity of the embryo, developing as it does near its caudal end
• Primitive streak is produced as a result of the proliferation & convergence of ectodermal cells & forms a linear thickening along the ectoderm

• From the primitive streak, cells invaginate between the ectodermal & endodermal layers of the embryonic germ disc
• These progressively form a 3rd germ layer in this intermediate position, which is called the mesoderm

• The mesoderm spreads throughout most of the embryonic disc as the intraembryonic mesoderm, thus converting it into a trilaminar germ disc

• The mesoderm continues to spread peripherally beyond the limits of the embryonic disc proper to give rise to the extraembryonic mesoderm
• This spreads progressively between the extraembryonic ectoderm & the extraembryonic endoderm to form a trilaminar ophalopleure

• Cranial extremity of the primitive streak presents a rounded expansion termed the node of Hensen (primitive node)
• From here, cells invaginate & migrate forming a rode of mesdermal cells called the notochord
• Extends as far cranial as the buccopharyngeal membrane

1. Paraxial mesoderm
2. Intermediate mesoderm
3. Lateral plate mesoderm

1. Buccopharyngeal membrane
2. Cloacal membrane

• Beginning cranially, transverse fissures appear in the paraxial region of the membrane to produce paired blocks of mesodermal cells, called somite
• Soon after formation, they disaggregate & move to respective areas of the embryo

1. Sclerotome
2. Myotome
3. Dermatome

• Extraembryonic mesoderm splits into 2 layers separated by a cavity- the extraembryonic coelom (exocoelom)
• This cavity spreads gradually to involve the lateral plate mesoderm to form the intraembryonic coelom

• The introembryonic coelom on either side spreads cranial to untie the midline, cranial to the buccopharyngeal membrane
• The entire intraembryonic coelom is horseshoe shaped
• The midline region is the future pericardial cavity
• The narrow intermediate region is the primitive pleural cavity

1. Ectoderm
2. Mesoderm
3. Endoderm

• Flat structure
• Undergoes development that transforms the flat structure into a tubular structure
  • This begins shortly after the first somites appear, initially in the head region

• One tube inside the other
• Outer tube: body wall (ecto & mesoderm)
• Inner tube: digestive tube (meso & endoderm)
• Space in b/w is the coelomic cavity

1. Notochord serves as the inductor for the formation of the head
2. Thickening of the ectoderm over the cranial end of the notochord to form the nueral plate (precursor of the brain)
3. Neural plate folds & forms the neural tube (later differentiates into majore parts of the brain)
4. Appearance of neural crest cells on dorsolateral sides of the neural tube

• Pigmented cells (melanocytes) of the skin
• Glial cells of the CNS
• Chromaffin cells of the adrenal medulla
• Neurolemocytes (Schwann cells) of peripheral nerves

• Head region forms first
• Most advanced part of body
• Specific modifications of the basic cylindrical pattern are 1st seen here

1. Cranial end of embryo grows up & forward
2. It folds back to form head fold
3. At same time, lateral folding of ectoderm on each side of embryo
4. At cranial end of embryo, ectoderm encompasses the underlying mesoderm
5. 2 lateral folds fuse together ventrally to create cylindrical outgrowth that is the head
   
   • Rostral end is closed because of forward growth
6. Inner tube is the foregut
7. Ectoderm covering the head is continuous with the rest of the ectoderm
8. Head is elongated by continued growth & folding
9. Caudal part of head incorporates the 1st several pairs of somites to form the musculo-skeletal strucutres of the head

• Embryo converted from flat disc to curved cylindrical structure
• Lateral & ventral body walls develop by folding of somatopleure
• Gut is formed by being nipped off the yolk sac & trapped w/in the developing body of the embryo
• Embryo is completely surround by the fluid filled amnion
• A group of structures are bunched together at mid-abdominal level (future umbilical cord)

• Several membranes which develop 'outside' the developing embryo (extraembryonic membranes)
• Serve protective & nutritive functions for the embryo

• Endoderm spreads around the inner surface of the trophoblast
• 2 layered wall is the bilaminar omphalopleure
• In domestic species, the yolk sac greatly enlarges as the whole blastocyst elongate

• Extraembryonic mesoderm spreads outwards from the trilaminar germ disc
• Progressively converts the yolk sac wall into a trimlaminar omphalopleure
• Blood vessels develop in this mesoderm & coalesce to form vitelline vessels

• The apposition of part of the yolk sac to the trophoblast (developing chorion) causes a temporary, localized placenta (the choriovitelline placenta)
• Remains functional longest in carnivores & horses
• Replaced later by the definitive placenta

• Develops from the trophoblast & outer layer of the extraembryonic mesoderm
• After this splits, its part of the extraembryonic portion of the somatopleure

• A very large sac in domestic species
• When fused with the allantois, its particuarly important in the development of the definitive chorioallantoic placenta

• Folds of the somatopleure develop, gradually overgrow the embryo, & fuse together
• The fusion completes the amnion & the chorion
• Concurrent folding of the embryo leads to teh union being carried ventrally to surround the embryo & form the outer lining of the umbilical cord
• Great accumulation of fluid causes the amnion to become a very large, fluid-filled sac

• Forms as a diverticulum of the hindgut
• Consists of an endodermal lining with a convering mesoderm
• Continues to grow into the extraembryonic coelom
• Enlarges greatly in size due to fetal urine (in domestic species)

• The allantois fuses with the chorion & the amnion
• The joint membranes are the allantochorion & allantoamnion

• Vessels associated with the allantois
• Umbilical arteries are large vessels arising from the caudal aorta
  • They ramify in the allantochorion
  • Provide the fetal vascular component of the chorioallantoic placenta

• The organ of fetal nutrition, respiration, & excretion
• Has fetal & maternal structural components
• Maternal components are specializations of the endometrium

• A combined group of structures which enter & leave the embryo at the ventral body wall

• Amnion

• Vitello-intestinal duct
• Stem of the allantois
• Umbilical vessels
• These all lie in loose embryonic connective tissue called Wharton's jelly

• A tubular structure connecting the future urinary bladder with the allantoic cavity
• Proximal part of the allantois
• Its connection to the future bladder marks the site of origin of the allantois from the early hind gut