3  Embryology of Farm Mammals

3.1 Duration of prenatal life

To keep things simple, we are going to call hatching - birth.  This will enable us to use prenatal to indicate events occurring before birth in mammals and before hatching in poultry, and postnatal for events after birth or hatching. The duration of the prenatal period is very important.  Obviously, the producer needs a good idea of when new animals will be born.  A more subtle point is the length of the prenatal period determines the cost of final products. To produce a new calf takes a certain length of time - a time during which all the costs of production must be paid by the producer. Most of the animal's live weight is obtained by postnatal growth. Thus, veal must be more expensive per kilogram than beef - the prenatal period is the same for both, but beef gets a magnifier effect from longer postnatal growth.   Lamb must be more expensive per kilogram than mutton.

Stage of development Beef
Forelimb bud
Hind limb bud
Prenatal duration %

3.2  Embryology

The embryo will develop from the inner cell mass  roofed‑over by amniotic folds - these later fuse. Endoderm from the inner cell mass spreads over the inner surface of the trophoblast and, at this stage, the blastocyst is said to be bilaminar. The blastocyst becomes trilaminar when mesoderm from the inner cell mass spreads between the outer trophoblast layer and the inner endoderm layer. The mesoderm layer then splits within itself to form a cavity within the mesoderm, the extra-embryonic coelom.



   The space within an avian egg is limited, and the growth of the foetus and allantois is balanced by shrinkage of the yolk sac. The mammalian uterus, however, is capable of considerable expansion, and the allantoic cavity grows quite large.


3.3 Cattle

3.4 Sheep

3.5 Pigs

3.6 Genetic engineering
   The experimental manipulation of prenatal development in farm animals has resulted in commercial methods such as embryo transfer. This early stage of animal development is also the most amenable stage for genetic engineering. Genes were first transferred experimentally into new locations by techniques such as chromosome assimilation and cell fusion. To show the transfer of genes actually had taken place, the recipient cell was placed in an environment in which the transferred genes would reveal themselves. For example, the recipient cells might suddenly acquire the ability to survive in a medium that lacked a hitherto essential nutrient. Other techniques also were developed for the transfer of isolated DNA: (1) direct injection into the nucleus of a recipient cell using a micropipette or high-speed impact, (2) the use of a virus to carry the DNA into the recipient cell, and (3) precipitation of the DNA with calcium phosphate to make it attractive for uptake by the recipient cell.

   The genetic engineering of mammalian development by such techniques has only just started and, at the present time, depends heavily on the use of relatively simple and extensively studied genes. Apart from important exceptions such as myofibre hyperplasia in double-muscled cattle, most of the commercially important traits of meat animals are regulated by large numbers of genes and are difficult to manipulate by genetic engineering. In laboratory animals, growth rates may be increased dramatically by introducing genes that lead to increased levels of somatotropic hormone (STH).  Whether or not genetic engineering at the molecular level will enable us to develop superior types of meat animals remains to be seen. The challenge is to make meat animals more efficient, not merely larger, and to enhance the quality of their meat - not to scare the meat-eating public towards vegetarianism.

Further information

Structure and Development of Meat Animals and Poultry, Chapter  8.