@JellySaurus
She says that all embryos begin as female.
That's what I was taught 35 years ago. But I was also taught to keep a person warm if they had a fever, and to put butter on burns.
Same here - several times - in Zoology A Level, then Human Biology at University and then in a University course on Embryology that we studied with medical students.
(About embryos - not fever and butter on burns 
)
I keep seeing people poo-pooing the idea but until very recently that is how it was understood that mammalian embryos developed, ie. the default was female.
This was also cited as an explanation for males being less likely to result in a live birth and being less likely to thrive and survive in early childhood, ie. due to the stress of the shift in the "natural" developmental pathway from female to male.
According to this article, the "default female theory" was only disproved in 2017.
It’s Hard Work Being a Boy (and, It Turns Out, a Girl)
SEPTEMBER 6, 2017
Basic biology tells us that in humans and other mammals, embryos with two X chromosomes develop as females, whereas embryos with one X and one Y chromosome develop as males. If only it were that simple. As it turns out, genetic makeup doesn’t fully determine which sexual organs will develop in utero.
Every developing embryo, irrespective of its sex, at one point contains both male and female reproductive tracts, referred to as the wolffian duct and the müllerian duct, respectively. If the fetus produces testosterone and the anti-müllerian hormone (AMH) gene products from the Y chromosome — these molecules elicit cellular signaling events that lead to the destruction of the female müllerian ducts. The wolffian ducts subsequently develop into male reproductive organs, such as the seminal vesicles, vas deferens and accessory structures. In the absence of testosterone or AMH, the wolffian ducts degenerate and the müllerian ducts develop into female reproductive organs including the fallopian tubes, uterus, cervix and upper vagina. These pathways were first identified and elucidated in the 1940s by the French endocrinologist Alfred Jost, who conducted intricate experiments using rabbits and showed that female development is a "default" pathway that needs to be actively overridden for the development of male sex organs.
A new study published in Science by Humphrey Yao, Ph.D. challenges this age-old concept of the female pathway as “default” and shows that the development of femaleness is also an active process. The authors implicated a protein called COUP-TFII as a key player that is required to actively eliminate the wolffian duct in a developing female embryo in order to give it female characteristics.
Continued at:
biomedicalodyssey.blogs.hopkinsmedicine.org/2017/09/its-hard-work-being-a-boy-and-it-turns-out-a-girl/