what is the first stage of an embryo
Human embryogenesis The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days.[2] During this stage, the zygote begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryogenesis continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow.
Heart development At around 18 to 19 days after fertilisation, the heart begins to form. This early development is critical for subsequent embryonic and prenatal development. The heart is the first functional organ to develop and starts to beat and pump blood at around day 21 or 22.[1] The heart begins to develop near the head of the embryo in the cardiogenic area.[1] Following cell signalling, two strands or cords begin to form in the cardiogenic region[1] As these form, a lumen develops within them, at which point, they are referred to as endocardial tubes.[1] At the same time that the tubes are forming other major heart components are also being formed.[8] The two tubes migrate together and fuse to form a single primitive heart tube, the tubular heart which quickly forms five distinct regions.[1] From head to tail, these are the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus.[1] Initially, all venous blood flows into the sinus venosus, and contractions propel the blood from tail to head, or from the sinus venosus to the truncus arteriosus.[1] The truncus arteriosus will divide to form the aorta and pulmonary artery; the bulbus cordis will develop into the right ventricle; the primitive ventricle will form the left ventricle; the primitive atrium will become the front parts of the left and right atria and their appendages, and the sinus venosus will develop into the posterior part of the right atrium, the sinoatrial node and the coronary sinus.[1]