67. In the avian embryo, the blastocoel-like fluid-filled cavity is formed between:
(1) epiblast and hypoblast
(2) hypoblast and yolk
(3) primary hypoblast and secondary hypoblast
(4) Koller’s sickle and Posterior Marginal Zone

Introduction

In avian embryogenesis, a critical structural feature is the blastocoel-like cavity, which functions analogously to the amphibian blastocoel but is adapted to the unique flat blastodisc morphology of bird embryos. This fluid-filled cavity forms between two essential cellular layers—the epiblast and the hypoblast—and plays a pivotal role in facilitating cell movements during gastrulation and proper embryonic patterning.

Structure and Spatial Arrangement

• The early chick blastoderm consists of two distinct epithelial layers: the upper epiblast and the lower hypoblast, both resting atop a large yolk mass.​

• The space that forms between the epiblast and hypoblast layers is the blastocoel-like cavity, creating a fluid-filled separation crucial for cell migration and embryonic development.​

• This cavity allows epiblast cells to freely move during the formation of the primitive streak and ingress to form mesoderm and endoderm.

Functional Significance

• The blastocoel-like cavity maintains physical separation between layers, ensuring correct spatial relationships for robust morphogenesis.

• By providing a fluid environment, it supports the initial differentiation processes and facilitates the regulated ingress of epiblast cells at the primitive streak during gastrulation.​

Other Options Overview

• The cavity is not formed between the hypoblast and yolk (Option 2); this space is called the subgerminal cavity, which is distinct from the blastocoel-like cavity.​

• It is also not between the primary and secondary hypoblast (Option 3) or between Koller’s sickle and posterior marginal zone (Option 4), as these are specific cellular regions with other functions.​

Summary Table: Location of Blastocoel-like Cavity in Avian Embryos

Conclusion

The blastocoel-like cavity in the avian embryo forms specifically between the epiblast and hypoblast layers, providing a fluid environment necessary for embryonic cell migration and layer formation. This cavity plays a critical role in supporting the morphological and developmental events characteristic of avian gastrulation.