【Science】5th Grade: The Birth of Living Things (Medaka and Humans)
Hello! Today, let's study together about the "beginning of life."
We will explore the mysteries of life, such as "How are medaka (Japanese rice fish) born?" and "How do babies grow inside their mother's womb?"
It might seem a little difficult at first, but if we break it down into key points, you'll be just fine!
What you will learn in this chapter:
1. The changes in medaka eggs and how they grow
2. How humans grow inside the mother's body
3. The commonalities in how life is passed on
1. The Birth of Medaka
First, let’s look at the medaka, a fish very familiar to us. Medaka increase their numbers by laying eggs in the water.
① How to tell males and females apart
To distinguish between male and female medaka, look at the shape of their "fins"! This is a very common test question.
- Male: The dorsal fin has a notch (cut), and the anal fin is large and square in shape.
- Female: The dorsal fin has no notch, and the anal fin is tapered (smaller toward the back).
💡 Pro-tip for remembering:
Remember that males have a "cool, jagged (notched) dorsal fin!" Since females carry eggs inside their bodies, their anal fins have a slightly more understated shape.
② Changes inside the egg (Growth of the fertilized egg)
When the male's sperm joins the egg laid by the female, it is called fertilization, and the egg is called a fertilized egg. As time passes, the fertilized egg changes as follows:
- Day 1: You can see many "oil droplets" inside the egg.
- A few days later: You can see the eyes.
- A few days after that: The heart starts beating, and you can see blood flowing.
- About 10–14 days: The fish starts moving around inside the egg, and eventually breaks the shell to come out. This is called "hatching."
Key Point:
Baby medaka (fry) are born with a "yolk sac (nutrient sac)" on their belly. Because of this, they can survive for 2–3 days after birth without eating any extra food!
⚠️ Common mistake:
People often think that medaka eat food greedily the moment they are born, but don't forget that they grow using the nutrients from their own sac at first.
【Summary of the Medaka Section】
Medaka tend to lay eggs when the water temperature is around 25°C. Their bodies and hearts form inside the egg, and they hatch and are born under their own power.
2. The Birth of Humans
Next, let's talk about us, "humans." Humans don't lay eggs outside like medaka; instead, they grow their babies inside the mother's body.
① Growth inside the uterus
A human baby grows in a place inside the mother's body called the uterus (womb). The inside of the uterus is filled with a liquid called amniotic fluid, which acts like a cushion to protect the baby from shocks.
② How do they get nutrients and oxygen?
A baby cannot breathe or eat meals while inside the womb. That's where these two things become important:
- Placenta: A special organ that connects the mother's body to the baby.
- Umbilical cord: The tube that connects the placenta to the baby's body.
How it works:
Oxygen and nutrients are delivered from the mother's body to the baby through the placenta and umbilical cord. Conversely, the waste products (like carbon dioxide) produced by the baby's body are returned to the mother's body.
💡 Analogy:
The "umbilical cord" is like a "magic straw" that delivers everything from the mother. Because of this, the baby can grow big without having to use their own mouth to eat.
③ Time until birth
For humans, it takes about 38 weeks (about 10 months) from fertilization until birth. While the baby is much smaller than a medaka egg at first, they grow to be about 3000g (3kg) by the time they are born.
Key Point:
As the baby grows, the mother's uterus expands significantly. Raising a life requires a very long time and a great amount of energy.
【Summary of the Human Section】
Humans grow inside the uterus, receiving nutrients through the placenta and umbilical cord, and are born only after they have fully developed.
3. Similarities and Differences Between Medaka and Humans
Finally, let’s compare what we have learned.
Similarities
・Both start from a fertilized egg.
・Both need nutrients and oxygen to grow.
・The body develops as the heart forms and blood begins to flow.
Differences
・Medaka grow outside the body (inside an egg), while humans grow inside the body (uterus).
・Medaka grow using only the nutrients found within the egg, while humans continue to receive nutrients from the mother.
Fun Fact:
Animals that are born from eggs, like medaka, are called "oviparous," and those born as live babies, like humans, are called "viviparous." You'll learn more about these in junior high school, but it's impressive if you already know them!
🌟 Conclusion
Whether it’s a medaka or a human, we are all the same in that we start as one tiny egg and continue the cycle of life.
Our own lives have been carefully nurtured over a long time just like this.
If you look at photos in your textbook or an encyclopedia and try to find things like "Where are the eyes?" or "Where is the heart?", you'll understand it even better!
Make sure to praise yourself for studying hard today. Great work!