Being placental mammals, we experience our inaugural breath upon birth. However, creatures hatching from eggs, like birds and reptiles, lack the luxury of an umbilical cord for oxygen supply. This raises the question: how do they manage to breathe? Take, for example, a developing chick within an egg. To unravel this enigma of respiration within an egg, we must first delve into the intricacies of the egg structure itself.
How Animals Breathe in Eggs
Bird eggs possess intricately textured shells, discernible under microscopic scrutiny, primarily composed of calcium carbonate. These shells feature no less than 17,000 minuscule pores, enabling the exchange of oxygen between the embryo ensconced within the egg and the external environment. Conversely, they permit the release of carbon dioxide and water from the egg.
Situated between the eggshell and the albumen (egg white) are two translucent membranes: the chorion and the allantois. These membranes serve as a formidable bulwark against bacterial intrusion while simultaneously providing a network of blood vessels.
Reptile eggs mirror their avian counterparts in some aspects, potentially bearing shells as delicate as parchment or manifesting in a softer, more leathery form. The majority of reptile eggs display a porous nature, accommodating the flow of air and water. They also possess a heightened affinity for water absorption compared to bird eggs. Nevertheless, the encapsulating membrane around a reptilian embryo doesn’t invariably attain the same completeness as that observed in avian embryos.
Notwithstanding these discrepancies, both bird and reptile embryos partake in the presence of the chorion and allantois, which function as respiratory organs that inaugurate the embryonic “breathing” process.
Curiously, the embryo of a chick dwelling within an egg navigates through three distinct phases of respiration. Reptiles, in turn, exhibit a similar process, albeit bypassing the intermediate stage to leap directly from the first phase to the third.
Certainly, until the fledgling chicks or developing reptiles reach the stage of lung maturity, their imperative need to intake oxygen and expel carbon dioxide persists. In placentals, which encompass humans and certain marsupials, this vital exchange is effectively orchestrated through the mother’s umbilical cord and placenta.
Conversely, avian creatures employ a rather mechanism for this gas exchange. Within the avian realm, this process involves a sophisticated interplay between several elements, including the diffusion of gases across the eggshell, a transfer channel from the external environment to the inner confines of the egg. This respiratory dynamic is facilitated by the chorioallantoic membrane, which encompasses both the chorion and allantois. In a similar fashion, reptiles also exhibit a chorioallantoic region, which functions as a respiratory apparatus.
In birds, the development of the chorioallantoic membrane commences approximately three days into the incubation period, maturing over the subsequent two weeks. This highly vascularized membrane operates as a conduit for the seamless exchange of oxygen and carbon dioxide.
Beyond its respiratory duties, this remarkable membrane assumes a pivotal role in the embryonic skeletal growth of avian species. It orchestrates the transportation of calcium from the eggshell to the burgeoning chick’s or reptile’s developing embryo. It is important to note that while this calcium contribution from the eggshell is the norm for most species, some reptiles do source a portion of their required calcium from the yolk.
During its in-egg phase, the embryo relies less on lung respiration. However, a pivotal divergence surfaces as the embryo approaches the moment of hatching, setting apart reptiles and birds. In the days preceding hatching, a chick adopts a distinct posture: it curls itself beneath its wing, its head elevated toward the eggshell’s apex, and its beak aligned in this direction. This strategic stance provides access to the air cell situated at the egg’s uppermost point.
The genesis of the air cell traces back to the egg’s laying. Freshly deposited eggs match the mother’s body temperature, but they promptly cool. As the egg cools, the inner shell membrane contracts and detaches from its outer counterpart, engendering a pocket of air—a nascent air cell. This cavity gradually fills with air from the external environment, drawn in through the shell’s pores, while the egg undergoes incubation.
Upon entering this air cell, the chick takes its inaugural breath, marking the activation of its lung function. The air cell sustains its air infusion through a process of diffusion. Even though diffusion through the chorioallantoic membrane is still effective, it takes a back seat to lung activity as the time for hatching gets closer. During this phase, if one were to press an ear against the egg, a faint “squeaky” sound might be discerned. Given that birds lack vocal cords, this sound emanates from a specialized structure known as the “syrinx.”
Conversely, most reptilian eggs eschew the air cell phase, progressing directly to the third stage of respiration.
Numerous creatures hatching from eggs exhibit a minute, pointed protrusion known as an “egg tooth” (technically termed a caruncle) situated on their beaks or nasal regions. This structure can be composed of hardened skin (as seen in crocodiles and birds) or a literal additional tooth (as observed in certain lizards and snakes). Its primary function lies in breaking open the egg’s shell. Following hatching, this appendage either falls off or is absorbed.
The positioning of a chick’s head beneath its wing positions it adeptly for pecking at the egg. Subsequently, the chick employs its egg tooth to crack open the shell. As the chorion and allantois membranes desiccate, their purpose wanes. At this juncture, the chick’s reliance shifts to its lungs for breathing.
Reptile hatchlings utilize their egg tooth, situated on their nose, to pierce through the weakened eggshell—its structural integrity diminished due to calcium absorption—and initiate their first breaths. Certain reptiles, including crocodiles, even generate sounds from within the egg. Unlike birds, these reptiles possess a larynx and vocal cords akin to those of humans.
In essence, birds and reptiles enact similar processes while enclosed within the egg. This correspondence should not astonish, given the close kinship between birds and select reptile species. These creatures all share a commonality in the form of specialized eggs that shield and furnish air to the burgeoning embryo.