Re: is it true that all of the embryos develope a penis then it goes away?

Do all embryos develop a penis?

Not exactly, it sort of depends upon what you mean by penis.

An important fact of embryological development is that males and females 
have exactly the same body parts. During development some parts get big, 
others stay small, some fuse together, others stay separate. It is the 
pattern of enlargement and fusion that creates the sex differences.

All embryos start with a genital tubercle. In pictures of a sufficiently 
immature fetus this tubercle may look like a penis. Some specialists 
identify this structure as the fetal “phallus.” In other contexts, 
phallus is just a fancy word for penis, which is probably why you got the 
impression that all embryos start with a penis. In biology, phallus can 
refer to the penis, but it can also refer to an enlarged clitoris, which 
is the normal condition in some other animals. The development of the 
genital tubercle is why ultrasound imaging cannot be used to determine 
the sex of a fetus before a certain stage of development – at this early 
stage both males and females looks alike.

In males, the genital tubercle fuses with the urethral swellings and then 
enlarges to become the penis. In females, the structures stay separate 
and do not enlarge as much to become the clitoris and the labia minora. 
So, its still all the same parts, just a different pattern.

References:

Sadler, TW (2004) Langman’s Medical Embryology. Lippincontt, Williams & 
Wilkins: Philadelphia.

Moore, KL and TVN Persaud (1993) The Developing Human. WB Saunders Co: 
Philadelphia.