Final answer:
The mesoderm generates most of the cells in the developing mammalian forelimb, while the ectoderm is responsible for forming the neural tube.
Step-by-step explanation:
The developing mammalian forelimb is primarily generated by the mesoderm, which is one of the three primary germ layers. The mesoderm is responsible for giving rise to the muscle cells and connective tissue in the body, including those present in the limbs. For the neural tube, its formation is credited to the ectoderm, another germ layer. The ectoderm specializes in forming the nervous system as well as the epidermis and other tissues.
Direction of embryonic development such as gastrulation and organogenesis are orchestrated by the three primary germ layers: the ectoderm, mesoderm, and endoderm. Each layer has a specific role in producing various tissues, organs, and systems necessary for building a complex organism.