On the fifteenth day, leadership began to emerge. The past few days had been busy ones for the cells of the embryo. Their house had grown greatly in both size and complexity, with construction proceeding at a feverish pace as all the cells found themselves inspired by the existence of the other world beyond their walls. It was beautiful; so enormous and well organized. Perhaps, if they all worked hard, they could one day build something like that too?

And their hosts were generous. The three girls they had seen before were apparently called red blood cells, and they and their friends provided the embryo with packages of food and oxygen at regular intervals. They never came to visit directly, saying they weren't allowed, but always stopped just outside the entrance to the tunnel and handed things over. One day, they'd even had a visit from a very pale cell who called himself a neutrophil. He'd seemed a bit suspicious at first, scowling at the damage that had been done to the wall of the blood vessel, but then relaxed once he saw that things were all in order. "Just a normal process," he'd muttered to himself as he walked away. "But why'd they have to be so messy about it?"

And now, something else new had happened to the embryo. A small group of cells had suddenly decided that they were the bosses of everyone else and were starting to direct others, telling them where to migrate and differentiate. They started digging a ditch right through the floor and installing stairs, saying there was no reason they couldn't start building downward as well as upward. And sure enough, something of a basement was forming. The place was practically three dimensional now, a house with multiple floors rather than a simple ball or a flat room as it had been in its previous stages.

"Hey you!" One of the organizers - a primitive streak cell, as she called herself - seemed to be pointing at PGC1.

"Me?" She asked curiously.

"Not you, him," the primitive streak cell said crossly, indicating a cell near PGC1 who was leaning against the wall and appeared to be taking a nap. "Tell him to stop snoozing and get over here, I've got his assignment ready."

PGC1 tapped the cell on the shoulder and he started awake - "Wha? What time is it, did I miss something?"

The primitive streak cell beckoned him over and pulled a marker out of a pocket in her smartly cut suit jacket, crossing out whatever number it said on his name tag and scrawling something else in replacement. "Looks like you're endoderm, so get migrating," she said, indicating in the direction of the steps leading down to the basement.

As the rest of the day progressed, more and more cells got their assignments. PGC1 watched carefully, waiting her turn. She noticed that at first the primitive streak cells were assigning almost everyone to a division called endoderm, who were sent to the basement. Later that day and into the next day, it became more common to see someone sorted into "Ectoderm" or "Mesoderm." The ectoderm cells got to stay where they were, while mesoderm cells went down into the basement where the endoderm cells were already toiling.

The more it happened, the more the sorting began to be a cause for celebration. Cells gathered around in small clusters, examining their new name tags and comparing assignments. PGC1 noticed that only the epiblast cells were getting these assignments, but the hypoblast cells were busy organizing their nutrient pantry in the yolk sac kitchen and seemed rather beyond it all.

PGC1 found a few others of her kind in the gathering crowd. "Where'd you think they'll put us?" She asked PGC3, who she spotted chatting with PGC9.

"Oh, you haven't heard yet?" asked PGC3 with a toss of her hair, and PGC1 remembered that this particular member of her progeny was a bit snobbish. Oh well, you couldn't choose your compatriots, and they were already talking.

"Guess I missed the memo. What's our assignment gonna be?"

"We're not going to get assigned to one of those silly germ layers. We've been told to gather at the edge of the yolk sac and await further instruction."

"Guess this world isn't ready for us yet," PGC9 quipped. PGC1 had never seen this particular germ cell before, but she noticed that he looked quite different than her even though he was supposedly her grand-progeny or something. Come to think of it, most of the other epiblast cells fell into a similar archetype, and until they'd started differentiating into their three lineages it had been difficult to tell one apart from another. But all the PGCs looked pretty different from each other. Their outfits were similar – boring white T-shirts and jeans. But there was a hint of uniqueness about them beyond the clothing. She herself was a brunette, as she could tell even without a mirror by the long straight hair that fell across her shoulders. PGC3 was blonde, and PGC9 was a boy with spiky black hair that looked like it needed some gel or a good brushing.

The three of them all looked at each other and shrugged, then began walking toward the kitchen. Somehow the distance to its doorway seemed like it had grown since yesterday, and their progress was made slower by the crowd of other cells milling around and talking excitedly. PGC1 noticed that by now a line had formed to get into the basement. This whole place was rather cramped, and PGC1 couldn't wait until it got bigger and became more like the other world beyond their walls.

When they got to the kitchen, the three PGCs saw that they were already late to the party. About a dozen other PGCs were standing or sitting against the nearest wall, and one of the primitive streak cells was standing near them and tapping her foot impatiently. All of them looked a bit like manic office pixies with their pencil skirts and dark eyeliner and frizzy hair pulled up in buns, so PGC1 couldn't really tell this one apart from the others.

"Finally, you're all here!" chirped the primitive streak cell excitedly. "You all are about to set out on the most exciting journey of your lives."

"Is there gonna be food?" asked a rather pudgy PGC who was already nibbling on a loaf of bread as he spoke.

"Of course, we'll keep you well supplied. You'll start off here in the yolk sac, after all."

"Where are we going?" Another voice called out.

"First, you'll wait here until gastrulation is complete," said the primitive streak cell, consulting a small notebook that she seemed to produce from thin air. "Then, you will set off along the wall of the yolk sac to the vitelline and into the rectum."

"And we'll wreck 'em," snickered PGC9 into PGC1's ear.

The primitive streak cell cleared her throat and glared daggers at him. "As I was saying," she continued. "After crossing the dorsal mesentery, you will colonize the gonadal ridge. Along the way, you'll multiply by mitotic divisions, so that there will be about a hundred of you by the time you arrive. But the gonadal niche only has room for a few progenitors, so you'd better sort that out amongst yourselves. Only the fittest survive, so may the odds be ever in your favor."

There was a long, pregnant pause while the cells mulled over this new information. PGC3 was the first to break the silence. "Uhh, I don't know if you know this cause you're so busy giving everyone orders… but like, none of those places exist."

PGC1 didn't necessarily agree with the other germ cell's tone, but she nodded in assent at the message. She had heard of the yolk sac, but literally none of the other places that had been mentioned. And… what was that about a competition? She didn't know if she liked the sound of that…

"They don't exist yet," the primitive streak cell sighed, a hint of impatience overshadowing her earlier excitement. "We're still building them, but rest assured they'll be up and running in a few days. Just stay here for now and prepare for the journey. We'll all be excited to follow your progress!"

"But… wait…" PGC1 ventured. "You said there was only room for a few of us. What happens to everyone else?"

All the PGCs exchanged nervous glances and… was it just her imagination, or was there a tinge of suspicion in some of them as well?

But the primitive streak cell had already turned about sharply and hustled away to her next destination. Aside from the hypoblast cells bustling around expanding the yolk sac, the PGCs were alone.

Sorry for the gap between updates! I was finishing up my semester of classes in online grad school, and then just this week they let me back in my lab to work part time again after quarantine ended. So I've been busy but will try to keep up with this story if I can, it's a good review of embryology for me as well.

FYI, the primitive streak is a line that forms along the back of the embryo around day 15-16 for humans, and it starts the process of gastrulation by signaling to the surrounding cells. It's the place where the cells ingress and travel down and around to the bottom and sides of the embryo, and turn from pluripotent stem cells into lineage specified cells of the ectoderm, endoderm, and mesoderm. I still have trouble visualizing it tbh but there are tons of good youtube videos and explanations.

Next chapter: Migration!