Everything happens quickly now. Until the 21st day the following things happen: the front gill arch (left and right) deepens itself and ultimately forms the auditory duct. The would-be eyes have a clear pigmentation and a duct for the olfactory organ deepens itself. The cerebellum is divided by a groove. The forelegs are now divided (as they are in the adult cat), with an internal skeleton, yet the hind legs are still primitive protuberances. The tail lengthens and is curled towards the body. Internally, veins differentiate for the blood circulation and nerves. The larynx, bronchi and lungs can also be discerned. Oesophagus, stomach and intestines are formed. Pancreas, thyroid gland, kidney and liver are also developed. On the side of the back, vertebrae are formed. The primitive spinal cord continues into the tail. Even the genital system is already present, albeit primitively.
The embryo is now 10mm to 24mm long. It may be clear that, especially in the early stages, people should be very careful with possible damaging substances. Vaccination with a live (attenuated) virus can be harmful to the embryo and also the use of, for example, remedies for fungi can be detrimental. Antibiotics are only to be administered if strictly necessary. Particularly Baytril should be avoided. Of course, the female cat's life takes precedence over the life of the kittens and naturally the female should only be mated if she is 100 percent healthy. But even pregnant cats can fall ill.
In the meantime the female cat shows signs that she is pregnant: her nipples become slightly larger and pinker than they were before. After 21 days, an experienced person can determine by way of palpation (a way of feeling and touching carefully) if the female cat is pregnant. At this stage, the foetuses are still so small (the size of a large pea), they can be distinguished among themselves and can therefore be counted. Some female cats suffer fr