ABSTRACT
The fundamental problem of development is how, as cells increase in number, each cell acquires the correct molecular composition at the correct time. An adult human has over 200 types of cells that are visibly different and hence differ in molecular composition. Cells from the anterior of early embryos gave rise to only anterior structures, such as parts of eyes, legs, and wings, whereas those from the posterior gave parts of the thorax and abdomen at the rear of the adult body. When the embryo has 64 cells, about 48 form the trophectoderm, a spherical outer layer one cell thick, which later forms extra embryonic tissues. Within this sphere lies the inner cell mass, only part of which gives rise to the mouse, the remainder again giving rise to extra embryonic tissues. The Drosophila egg nucleus first subdivides without cell division to give rise to several thousand nuclei.
