Introduction to the developmental processes during the third week (stages 6 to 7; ca. 17-19 days)
This chapter is devoted to the most important events during the embryonic period. All of the details of organogenesis were thoroughly treated in the corresponding chapters.
The third week of embryonic development was discussed in the trilaminar embryonic disk module. We come back to it here briefly in order to speak about angiogenesis and the formation of the embryonic blood circulation system
The angioblasts already appear early in the mesoblast (stage 5). The first blood vessels (angiogenesis) become visible in the third week in the splanchnopleura that adjoins the umbilical vesicle and the allantois (stage 7). They are also found in the extraembryonic part of the somatopleura at the level of the body stalk (stage 6) and again in the vicinity of the tertiary villi of the placenta (stage 6b).
The endocardiac tubes form in the middle of the third week in the cardiogenic zone above the prechordal plate.