?? ??? ??? ??? ??? ???? ?? ?? ??? ??? ???? ???? ???? ?????? ?????.??? ????? ??? ??? ??? ????? ?? ??? ????? ?????. ?? ??, ?? ??, ???? ?? ? ??? ??? ?? ???? ?? ???? ??? ???? ???? ?????.??? ????, ??? ??? ? ?? ?? ?? ????, ??? ????? ? ?? ?????.??? ?? ???? ????, ??? ??? ?? ???? ???? ?? ??? ?? ?? ??? ??? ??? ???? ??? ?? ????.???? ???? ?? ?? ? ? ?? ??? ???

All through embryonic development the cells from the interior cell mass continually divide and turn out to be extra specialised. As an example, a portion of the ectoderm from the dorsal Portion of the embryo specializes as 'neurectoderm', that can grow to be the future central anxious system (CNS).??? ???? ?? ??? ???? ??? ?? ??? ???? ?? ??? ???? ??

? ?? ???? ???? ????? ???? ? ????. ? ??? ??? ?? ???? ????? ??? ??? ??? ??? ???.???. ??? ????? ???? ??? ????? ???? ??? ?????? ?? ???? ??? ??? ????? ???.?? ?? ??? ???? ?? ??? ??? ???? ??? ??? ??? ??, ?? ??? ??? ?? ? ?? ??? ?????. ??? ???? ??? ???? ? ?? ??? ???? ??? ??? ??? ? ??? ?? ???? ??? ??? ??? ??? ??? ??????. ?? ??? ? ? ?? ???? ????????. ?????.

?? ??? ????? ???? ??? ???? ????. ??? ?? ??? ???? ????? ?? ????, ??? ??? ????, ?????? ?? ???? ????? ??? ????? ??? ???? ????.? ??? ???? ????, ????? ?? ?? ???? ?? ??????? ???????? ?????. ?????.?? ?? ??? ???? ??? ?? ??? ?? ??? ??? ???? ??????? ???? ???? ?? ?? ??????? ??? ??? ???? ???? ???? ?????? ????.?? ????? ???? ?? ?? ??? ?? ????? ??? ??? ???? ?????

??? ?? ?????? ???? ??? ??? ???? ???, ??? ??? ??? ???? ??? ??? ?? ? ???? ???? ?? ??? ??? ?? ??? ??? ?? ????? ? ????? ????? ???? ??.???? ????? ???? ???? ??? ?? ???? ??? ? ????:??? ????? ????? ????? ????? ????? ????? ????? ????? ????? ????? ????? ????? ?????? ????? ??????? ?????? ??? ???? ??? ??????? ????? ?? ??? ?? ??? ???, ? ???? ?? ???? ??? ? ?? ??