Tuesday, May 16, 2006


Placental mammals (subclass Eutheria) are highly developed mammals which give birth to young at a relatively advanced stage. There are almost 4,000 known species of placental mammals. The dominant ones are rodents and bats than other types of placental mammals. They are more advanced than the young of the other mammals, the monotremes and marsupials. Before birth, the young are nourished through a placenta. The placenta is a specialized embryonic organ that is attached the mother's uterus and delivers oxygen and nutrients to the young. Placental mammals are given this name because the placenta connects the growing embryo within the uterus to the circulatory system of the mother. It is the pathway provider for the nourishment of the foetus.

Marsupials are animals that belong to the order Marsupial and an infraclass Metatheria. The members of the class include kangaroo, koala, Tasmanian devil and the Virginia opossum. Marsupials are known to be the subdivion of mammals but they have a lot of characteristics. The marsupials are accepted as non-placental mammal. The female of these marsupials carries the young in the marsupial or pouch. This is where the young are provided with warmth and proper environment. They do have placenta which is non-invasive and functions in the transferring of nutrient and waste for limited period of time.

The different between the placental mammals and the marsupial are seen mostly through the mode of reproduction. The breeding season for the marsupial, takes place during December and continue through October and most of the infants are born between February and June. During mating the male attracts the female by making clicking sound by the mouth. The oestrous cycle of female opossum takes about of 28 days. During this period mating can take place. The sperm in the male opossums reproductive tract paired and then move through the lateral vaginal canals and become separate again in the female's tract. If fertilization occurs, the fertilized eggs move from the oviducts to the uteri where they will gestate for a short period before being delivered through the medial vagina or central birth canal. These normally give birth to infants weighing at 0.13 grams.

At birth, the infants make a long and difficult journey from where they got birth into the pouch and continue their development. The mother helps by licking the hair leading into the pouch, providing a moist path for the infants to follow.
The young remain in the pouch for about 2 1/2 months and at around 55-70 days the eyes open. As they become too large to fit in the pouch, they climb onto the mother's back and are carried as she searches for food. At this time the young are learning survival skills such as finding food sources and predator avoidance. If one of the young becomes separated from its mother it will make sneezing sounds to call her. She, in turn, will make clicking sounds.

The offspring of the placental mammals are born fully developed. This is one of the characters that distinguish this two. The placental mammals have a longer gestation period which results to the situation where off springs are able to develop fully. The extended maturation of placental is caused by the placenta which allows nutrients to travel from the mother's system to the embryo and waste to be carried away. The placenta is composed of several layers and these layers are supplied with blood and serve as immigration barrier letting nutrients to pass through. The embryo and the mother do not share the same blood supply


1. Northwest creation network, Marsupial evolution and post flood migration [Internet] [cited 2006 May 11] Available from:

2. Wgbh education foundation, convergence, marsupials and placental, 2001 [Internet] [cited 2006 may 11] Available:

3. Opossum society of the United States, Opossum reproduction and life cycle, 2003 [Internet] [cited 2006 may 11] Available:

Mr Lufuno Mukwevho
CSIR Pretoria
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Tel: (012) 841 2133
Fax: 012 842 7024.
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Email: lmukwevho@csir.co.za
My bloger URL: http://mukwevholufuno.blogspot.com


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