Webrelate the different stages of development to the adult structures of the central nervous system. Explain the expansion of the ventricular system of the adult brain from the. Webcorrectly label the following external genitalia of the developing male and female embryo. In humans, when do the secondary sexual characteristics first appear? Webthe embryonic stage lasts through the eighth week following fertilization, after which the embryo is called a fetus. The embryonic stage is short, lasting only about seven weeks.