Chapter 26 The stages of in utero development which follow fertilisation and the formation of a zygote can be arbitrarily divided into two phases, the embryonic stage and the foetal stage. The embryonic period is defined as the interval from fertilisation to the development of organ primordia. This interval in sheep, pigs and dogs is approximately 30 days, whereas in humans, horses and cattle the interval extends up to approximately 56 days. Data related to the stages of development from zygote formation to implantation are presented in Table 26.1. Table 26.1 Time in days, estimated from ovulation, at which early stages of embryological development occur in domestic animals, from zygote formation to implantation. Developmental change during the embryonic period is rapid and by the end of this stage, the primordia of most organs are established. The embryonic period is especially important as the differentiating cells which give rise to organ primordia are particularly susceptible to adverse genetic influences and deleterious external factors during this stage of development. The foetal period, which extends from the end of the embryonic period to parturition, is characterised by the growth and initiation of physiological functioning of body systems. In order to compare the rate of development among different species and the effect of drugs, radiation and environmental factors on such development, it is necessary to record normal developmental features of the embryo and foetus for defined stages of gestation. Such information is of value for estimating the age of embryos collected from abattoirs for research purposes or of aborted foetuses. Data for domestic species have been compiled which record the stage of pregnancy coinciding with features such as total length, crown–rump length, somite number, presence of ossification centres and the appearance of external features such as eyes, ears, limbs, teeth and hair (Figs 26.1 to 26.6
Age determination of the embryo and foetus
Animal
Two‐cell stage
Four‐cell stage
Eight‐cell stage
Morula formation
Blastocyst formation
Emergence of blastocyst from zona pellucida
Time interval during which implantation occurs
Cats
3
3.5
4
5
6 to 7
9
12 to 14
Cattle
1
1.5
2.6
6
7 to 8
9 to 11
17 to 35
Dogs
4
5
6
7
8
10
14 to 18
Goats
1.6
2.5
3.5
5
6 to 7
7 to 8
15 to 18
Horses
1
1.5
3.5
4.5
6 to 7
8 to 9
17 to 56
Pigs
1
2
3
4
6
6 to 7
12 to 16
Sheep
1.5
2
3
5
6
7 to 8
14 to 18
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