The Placental Mammal's Ova: An Egg in Which Yolk Is Absent

November 26, 2025 •

3 min read

Placental mammals are born with a finite number of ova at birth, and their eggs are a key example of one in which egg yolk is absent. This is a sharp contrast to the eggs of birds, reptiles, and fish, which are all defined by their rich, yolky interiors. The absence of yolk in these tiny eggs is an evolutionary adaptation.

Quick Summary

Placental mammals, including humans, produce eggs known as alecithal ova that contain no yolk, as the embryo receives nutrients directly from the mother via the placenta. Accidental, yolkless chicken eggs also occur.

Key Points

Alecithal Ova: Placental mammals, including humans, produce alecithal eggs that are naturally devoid of yolk, relying instead on the placenta for nutrient exchange.
Evolutionary Adaptation: The transition from egg-laying to internal gestation and placental development in mammals eliminated the need for a large, external nutrient source like yolk.
Fairy Eggs: A specific, anomalous case of a yolkless egg is the "fairy egg," occasionally laid by chickens due to a reproductive system error.
Nutrient Source: While mammalian ova contain a nutrient-rich cytoplasm (ooplasm) for early development, the primary, long-term nutrient source comes directly from the mother.
Comparison with Other Eggs: Unlike the tiny mammalian ovum, eggs from birds, reptiles, fish, and amphibians contain varying amounts of yolk to support embryonic growth outside the mother's body.

The question of an egg in which yolk is absent is answered by the reproductive biology of placental mammals. Unlike birds and reptiles that lay nutrient-rich, yolk-filled eggs to support their offspring, placental mammals, such as humans, dogs, and cats, have evolved a different strategy. Their eggs, or ova, are classified as 'alecithal' or 'microlecithal,' meaning they have either virtually no yolk or very little. The developing embryo receives its nourishment directly from the mother through the placenta, rendering a large, external yolk sac unnecessary.

The Structure of the Mammalian Ovum

The mammalian ovum is a microscopic, non-motile cell, significantly smaller than the yolky eggs of oviparous animals. Its anatomy reflects its unique method of receiving nutrients. The ovum is composed of the following key parts:

Ooplasm (Cytoplasm): This jelly-like substance surrounds the central nucleus. In mammals, it contains minimal nutritive material, relying on external sources after fertilization. The ooplasm provides sufficient sustenance for the early-stage embryo, known as a blastocyst, until it can implant in the uterine wall.
Zona Pellucida: A protective, transparent, and non-cellular membrane that encases the ovum. It is partly secreted by the ovum itself and partly by surrounding follicular cells. This layer is crucial for preventing multiple sperm from fertilizing the egg.
Corona Radiata: An outermost layer of nourishing follicle cells that remain attached to the ovum after it is released from the ovary. These cells provide vital proteins and other nutrients to the ovum, acting as a supportive cushion during its journey.

Evolutionary Shift: From Yolk to Placenta

The evolution of placental reproduction in mammals represents a major divergence from other vertebrates. For species that lay eggs on land (amniotes like reptiles and birds), the yolk-filled egg, or 'cleidoic egg,' was a critical adaptation for terrestrial life. This hard-shelled, self-contained unit provided all the water, salts, and nutrition needed for the embryo's development, protecting it from desiccation.

In contrast, placental mammals evolved to retain the embryo inside the mother's body, where it could be continuously supplied with nutrients and oxygen through the placenta. This strategy reduced the maternal energetic investment in producing a massive yolk store per egg, shifting the investment to a longer gestational period and direct nutrient transfer. This means that for placental mammals, the evolutionary pressure to produce yolky eggs was eliminated.

Accidental Yolkless Eggs: The 'Fairy Egg'

While the ova of placental mammals are naturally yolkless, birds sometimes produce yolkless eggs due to an anomaly in their reproductive system. These are often called "fairy eggs," "fart eggs," or "wind eggs".

Cause of the Anomaly: A fairy egg is formed when a piece of reproductive tissue or a small foreign body enters the hen's oviduct. The oviduct's shell-gland is stimulated to produce the egg white and shell layers around this small nucleus, even though no yolk was released.
Occurrence: These are typically very small and can be laid by young or older hens, or those under stress. They are a quirk of the hen's reproductive cycle and do not pose a health concern for the hen or for human consumption.

A Comparison of Egg Types by Yolk Content

This table illustrates the wide range of strategies employed by different animal groups regarding yolk content in their eggs.


Egg Type	Yolk Quantity	Examples	Nutrient Source
Alecithal	Absent or trace amounts	Placental Mammals (Humans, Dogs)	Placenta provides direct nourishment from the mother.
Isolecithal	Little, evenly distributed	Echinoderms, Amphioxus, some Invertebrates	Embryo relies on internal, intracellular yolk reserves.
Mesolecithal	Moderate, concentrated at one pole	Amphibians (Frogs, Salamanders)	Yolk provides nutrition until larva can feed independently.
Telolecithal	Large, concentrated at one pole	Birds, Reptiles, Fish, Monotremes (Platypus)	Large yolk sac provides all nutrients for development until hatching.

Conclusion

The most definitive example of an egg in which yolk is absent is the microscopic ovum of placental mammals. The evolution of the placenta as the primary source of nutrients for the developing embryo made a large yolk store obsolete. While birds can occasionally lay an accidental, yolkless "fairy egg" due to a reproductive system malfunction, this is a rare anomaly. In contrast, reptiles, birds, amphibians, and fish all rely on varying amounts of yolk to sustain their developing offspring, with the amount of yolk correlating to their nutritional strategy and evolutionary history.

For more detailed information on the structure and function of the mammalian ovum, you can consult sources like the Cleveland Clinic.

Frequently Asked Questions

An alecithal egg is one that contains very little or no yolk. This egg type is characteristic of placental mammals, such as humans and dogs, where the developing embryo is nourished directly by the mother's placenta.

No, chicken eggs that are typically sold for consumption are not naturally without yolk. However, a hen can occasionally lay a small, accidental, yolkless egg, often called a 'fairy egg' or 'fart egg,' due to a reproductive glitch.

Mammalian eggs don't need yolk because the embryo is retained inside the mother's body and is supplied with all necessary nutrients and oxygen via the placenta. This evolutionary development replaced the need for a large, external nutrient store.

An alecithal egg has little to no yolk and is found in placental mammals, while a telolecithal egg has a large quantity of yolk concentrated at one pole and is typical of birds and reptiles.

No, not all mammals. The monotremes, such as the platypus and echidna, are egg-laying mammals that produce large, yolky eggs similar to reptiles. This differs from placental mammals, which are the ones that produce yolkless ova.

The ooplasm is the cytoplasm of the mammalian ovum. It contains a small amount of nutrients that support the embryo during its very early stages of development, before it implants in the uterine wall and the placenta takes over.

Yes, it is perfectly safe to eat a yolkless 'fairy egg' from a chicken. It is simply an egg produced without a yolk due to a biological mistake, and it is otherwise just egg white and shell.