Stages in Prenatal Period and Twining in animals
Stages in Prenatal Period
Period of the ovum
In which male and female germ meet, unite and form the zygote which develops into the blastula and gastrula covering the first 14 days of intra uterine life.
Period of the embryo
From end of the second week, it extends to the eighth week. Differentiation of embryonic parts is completed. Placenta is formed, embryo shows resemblance to parents.
Period of the fetus
From the ninth week to the end of gestation.
Twins and Twinning
Twins are either identical twins or fraternal twins. In uniparous animals usually only one ovum is fertilized at a time. If two individuals are derived from a single fertilized egg, each member acquires the same chromosomal heritage and hence the similarity between the twins is so striking in physical, mental and other characters. They are identical, true or duplicate twins. They are always of the same sex.
Two or more individuals, derived due to independent ripening of two or more number of ova and fertilization, have the same, degree of family resemblance, as between brothers and sisters of different ages. They can be of the same or opposite sexes. They are fraternal twins. In animals, e.g. the cow there is identical twinning.
Also in bovines, a condition called Freemartin is recorded, which is the female fraternal twin of a male. It is sterile, as the internal genital organs are of the male type and the external genital organs are of the female type. This is caused by placental anastomosis and the influence of the male hormone which is earlier to appear in circulation.
