Everyone's freaking out about how seastars are born, but if you think that's weird, let me tell you about HUMANS (hint: we're more similar than you think) [a thread]... https://twitter.com/RebeccaRHelm/status/1356610600743354371

First, let's talk stars. The sea star begins life as a tiny embryo. This embryo grows into a little body complete with tissues & organs that looks a bit like a fairy, called a brachiolaria. But it does so AROUND a small inner pocket of super special cells called the "rudiment"...

At first, the rudiment doesn't do much, the brachiolaria does all the important work: swimming, collecting food, and most importantly, NURTURING the rudiment. BUT, over time...

the rudiment grows, first, it's very small, but as it gets bigger a little star-shape forms. Eventually, it gets SO BIG the fairy-like brachiolaria will sink, and the tiny star will pop out, leaving the brachiolaria behind. So, now let's talk about YOU...

Your first skin was very much like the fairy-shaped brachiolaria: outside of your adult body. As an embryo, you grew this skin to burrow into the lining of the uterus and to protect and nurture you. The rudiment that you now call YOU was only...

...a small interior pocket of super special cells, called the inner cell mass. No fingers. No toes. But your first skin was GROWING. It wasn't just a layer of cells anymore, it grew its own ORGAN...

Your first organ was your placenta, which you developed outside your adult body, but which was VERY MUCH part of you back then...

Your first body enveloped you, just like the fairy-like brachiolaria. Your skin was called your amniotic sac, your organ the placenta. This body was made of YOUR CELLS. It WAS YOU. THIS is a picture of the WHOLE YOU. But, in order to be born...

You had to break out of yourself. You ripped through your outer body when you were born, leaving it behind just as the young star leaves its old body when it settles to the seafloor...

So yes, sea stars are weird, but, and I mean this with absolute awe and amazement: so are we. [end of thread]

p.s. also I wrote an article about this for Mother’s Day a while back because placentas are https://slate.com/technology/2014/05/science-for-mothers-day-disturbing-biology-of-the-human-placenta.html