Ans. is 'b' i.e., 3 weeks Preembryonic phase of developmento Preembryonic phase of development extends from day zero to 3 weeks (days 0-21).o This phase of development can be further subdivided into :-Fertilization and implantation (first week of development)First week of development begins immediately after fertilization, Changes during first week include -Cleavage of zygote - 0-3 days.Formation of morula, transportation to uterine cavity and conversion into blastocyst - 3-5 days.Implantation - 6-7 days.Stage of bilaminar germ disc (second week of development)The stage of bilaminar germ disc formation approximately occurs during second week of development.On 8th day after ovulation the embryoblast is converted to a bilominar disc consisting of following twolayers -Epiblast, the upper layer.Hypoblast, the lower layer.At the end of 2nd week -i) Some of cubical cells of hypoblast become columnar and form prochordal plate.Soon after the formation of prochordal plate, the primitive streak is formed by the proliferation of cells of epiblast.During this period following changes occurs :During 2nd week trophoblasts differentate into cytotrophoblast and syncytiotrophoblasts.Formation of amnion and amniotic cavityDevelopment of yolk sac and extraembiyonic mesoderm.Development of chorion.3. Stage of trilaminar germ disc (third week of dev elopment)The stage of trilaminar disc formation is marked by changes during third week of development.At the beginning of third week there occurs, the most characterstic event, the gastrulation, which establishes all three germ cells, changing embryonic disc from a bilaminar to a trilaminar structure. During this period, embryo may be referred as to gastrula.Other events during this period areFormation of primitive groove.Formation of germ cells.Formation of notochord.Spread of mesoderm and formation of buccopharynyeal and cloacal membrane.Appearance of allantois.Formation of neural tube (neurulation).Development of somites.Formation of intraembryonic colon.Formation of primordial cardiovascular system.