Before that time, trophoblasts produce gonadotropin, a hormone that ensures that the endometrium is ready for implantation of the embryos. The embryos have nestled themselves for the next, very important phase: gastrulatio. Gastrulatio is an exceptional process. Different zones of the blastula fold, and form three more or less distinguishable layers: the ectoderm, endoderm and mesoderm. Ectoderm is the designation for the outer cells of the body: therefore, the skin (epidermis), but the central nervous system also has an ectodermal origin. The endoderm consists of cells that (will) make up the digestive tract, the gastrointestinal system, then. The mesoderm, finally, are the cells, which will make up the muscles, skeleton and internal organs. The first mesodermal structure that will be formed is the chorda, the spinal column. Meanwhile, growth continues and the embryo becomes egg-shaped. The dimensions are approximately 1.5mm by 1mm.
Gastrulatio is a vital and vulnerable moment in the existence of an embryo. At this stage, the genetic material of the embryo itself is expressed. Before that time, processes were mainly controlled by maternal influences (by the mother, therefore) through the material in the egg-cell, originating from the mother. Many embryos die at this stage because of lethal (deadly) gene combinations, producing non-functioning or defective proteins. At this stage the cat's embryo is 1.5mm by 1mm.
During the next stage the organs are developed. The ectoderm forms a skin that covers the entire body and a thickened plate develops, which folds into a groove: the neural crest. Ultimately, this will become a tube, which will submerge below the skin's surface and will differentiate to the central nervous system. We have already (or just!) reached day 13.