How to Build an Animal | Part Two

There really is no shortage of ways to build an animal, but there are rules to be followed.

In part one of How to Build an Animal, you learned about how one way of tracing back the connectedness of all animal life is to look for shared traits. Sure enough, there are some super-specific characteristics of animal life that are definite markers of 1) what makes an animal an animal, and 2) when different groups of animals split in the eons-long journey of evolution.

We took a close look at three types of symmetry and the kinds of animals that sport them and got oriented with the basic directions of cephalization. Now, it’s time to dive into how all of these things develop in the first place.

What better place to start than the beginning?

But before that, I want to take a moment to acknowledge that this topic is absolutely loaded with terms and concepts that are kind of hard to picture. That’s why I’m embedding one of my all-time favorite videos showing a timelapse of a salamander growing from a single cell. I highly recommend giving it a watch both before and after reading. It’s truly spectacular!

Development

If there’s a way to make babies, nature has figured it out, both sexually and asexually. We’re going to focus in on the sexual type. It typically goes like this: males produce a special type of cell with half the amount of genetic information as a normal cell called sperm. Females do the same, but we call their special cells eggs.

Like these frog eggs!

Yet, even still, some animals (take earthworms for example), are what we call hermaphrodites. These animals produce eggs and sperm at different times. They still need another member of the species to reproduce, but the role they play, or what they provide, can change.

On a related note, snail sex has got to be one of the most fascinating examples in nature and I’m absolutely going to go off at length about it at some point in the future.

I mean, how do you not look at this and have tons of questions??

After a sperm penetrates an egg and the good ole swapping and merging of genetic info begins, we say the egg has been fertilized. Now we get to call that fertilized egg a new name—Paul.

Just kidding, it’s a zygote.

Now, what follows is going to be an “in most animals” type thing. Is there variation to the formula? Oh, absolutely, and it’s amazing. For now, let’s keep it general.

To back up for a sec, when cells *want to make more cells, they have to undergo a process called mitosis. It’s a whole process that I’ll explain at some later point, but here’s the gist: cells copy themselves, nearly identically, which means one cell can become two, those two become four, then 8, then 16, and so on.

*cells of course don’t want to do anything. They just do and be.

A zygote is no different…yet.

When the zygote first divides into two total cells, we change its name yet again. Now it’s Blake.

Okay, kidding again.

Now it’s an embryo.

Those cells keep dividing until they form a solid little ball of cells full of fluid called a blastula.

This whole time, the total number of cells is increasing, but the total amount of cytoplasm isn’t. Weird, right?

This means that even though there are more cells, the embryo is remaining roughly the same size…for now.

In some animals, the outer blastula is just one single layer of cells. In others, it can be multiple layers. Either way, it the cells that make up the blastula continue to divide. Some of the cells move inward to form a new structure called the gastrula. It’s 2 layers of cells thick, sort of a sac, with an opening on one end.

It sort of looks like a double bubble, one inside another.

The inside ball will develop into a new animal. The large ball becomes the yolk sac. We’ve all seen this whenever we’ve cracked open a chicken egg!

Tissue Development

Remember the gastrula is two cell layers thick. The inner layer is called the endoderm.

That layer will develop into the digestive organs and the lining of the digestive tract.

The outer layer of gastrula cells is called the ectoderm.

That layer will develop into the nervous tissue and skin.

In some animals, cell division happens within the gastrula in another layer of cells called the mesoderm, forming between the ecto- and endoderm.

There are a few strategies for developing this layer depending on what kind of animal we’re talking about. Either way, mesoderm cells develop into muscle, the circulatory system, the excretory system, and sometimes the respiratory system.

This is the point at which things start to go down very different paths. There may be some similarities, but all-in-all, the ectoderm, endoderm, and mesoderm will go on to develop as they will for whatever animal they belong to. But even with all that range, there are still some things, deep down, that connects us all.


Phew, that’s a lot of new information.

But guess what? We’re one step closer to kicking off Class! Next time on How to Build An Animal, we’ll discuss the three types of body cavities found in animals, why they matter, some specifics to their development, and then…

Well, you’ll just have to wait and see!


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The Wild Life was created in January of 2017 by, me, Devon Bowker (He/They) after finishing my degree in wildlife biology. It’s been amazing to see how things have changed over the past 5 years, both personally and here. I have tons of ideas and projects in the works and cannot wait to share them with you. Whether you’re a long-time follower or new to The Wild Life, thank you for being here.

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