After a sperm has penetrated an ovum in the process of fertilization, entry of further sperms is prevented by

Tertiary egg membranes are lacking in mammals with the exception of the

In mammalian sperm, spirally arranged mitochondria are present in

The fluid filled in the blastocoel cavity of blastula is

If the ectoderm from neural tissue area is removed from the embryo and transplanted in place of presumptive belly ectoderm embryo will develop