PRACTICAL 4
ORGANOGENESIS
Learning Objectives.
1. Dissection of foetus to identify developing organs and to observe the
progressive normal development of the gross anatomy of various organ
systems in the sheep.
2. Observe development of the organ systems in different species, the pig and
cow.
3. To observe the foetal structures in the cardiovascular systemsthe umbilical
blood vessels, ductus sinosus in the liver, the ductus arteriosus and the
foramen ovale. To understand the changes these organs undergo at birth.
4. To observe and understand the pattern of morphogenesis in the gastrointestinal
tract and the looping of the intestines.
5. To recognise foetal organs and the transitional stages in the urogenital organs’
the mesonephros, metanephros and the external genitalia.
I. DISSECTION OF THE FOETUS
(i).
Examine the external features of the embryo and make a rough estimate
of its age (measure the length from crown to rump).
(ii). Examine the umbilical cord. Trace the umbilical vessel into the foetus.
Here are two umbilical arteries and two umbilical veins in the sheep. The
veins fuse at the umbilical ring.
(iii). Open the abdominal cavity:
 Trace the umbilical vein to the liver; follow the fused umbilical veins into
the DUCTUS VENOSUS in the liver.
 Trace the umbilical arteries to their origin
 Examine the digestive tract observing the divisions of the ‘stomach’, and
the intestines.
 Examine the urogenital system, noting in particular: (i) the testes or
ovaries, (ii) the kidneysmesonephros and metanephros. Observe the
mesonephric tissue overlaying the metanephros. Note also: the ureter and
bladder, urachus, the internal genital ducts and the external genitalia.
Identify the sex.
 Open the thoracic cavity. Examine the great vessels of the heart, noting
the DUCTUS ARTERIOSUS connecting the pulmonary artery with the
aorta. Why this connection? Remove the heart with the great vessels
attached, open the atria to expose the atrial septum and with a blunt probe
carefully search for the FORAMEN OVALE. What is this function of
the foramen, and what is its duration?
II. PROSECTED SPECIMENS.
1. Cardiovascular and respiratory system
2. Digestive system
3. Nervous system
4. Urogenital system-->urinary and reproduction
Makai-kai
June 2012
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