Final answer:
Major organs in human gestation begin to develop during the gastrula stage of the embryonic period, which is marked by the establishment of the primary germ layers through the process of gastrulation.
Step-by-step explanation:
The major organs begin to develop during the organogenesis stage of human gestation. Organogenesis occurs during the embryonic stage of development, which starts at implantation and lasts until the eighth week after fertilization. During organogenesis, the three germ layers (endoderm, mesoderm, and ectoderm) differentiate and form the foundation for all organ systems in the body.
Major organs in human gestation begin to develop during the gastrula stage. This occurs after the processes of fertilization, cleavage, and the formation of the blastula. During the gastrula stage, the cells from the blastula rearrange to form three germ layers: the endoderm, mesoderm, and ectoderm, all of which will differentiate into different organs and tissues through the process of organogenesis. Indeed, the heart starts to form in the mesoderm shortly after gastrulation, beginning to function even as it continues to develop, signaling a critical stage in organ development.
Following the gastrula stage, neurulation starts with the formation of the neural plate, which folds inward to become the neural tube, eventually forming the central nervous system. Thus, among the options provided, major organ development begins in the gastrula stage (b).