Placenta is formed from

In the development of the human body,the ectoderm is responsible for the formation of:

Which of the following is not a correct statement about the umbilical cord?

Which of the following cannot pass through the placenta into the foetus?

After one month of pregnancy,the embryo's $A$ is formed. By the end of the $B$ month of pregnancy,the foetus develops limbs and digits. By the end of $C$,most of the major organ systems are formed,for example,the limbs and external genital organs are well-developed. By the end of $D$,the body is covered with fine hair,eyelids separate,and eyelashes are formed. Here $A$ and $D$ refer to:

The type of blastula formed in birds is