Developmental Anatomy

Stage 5: Lab Manual - Horizon 5 Ovum Implanted But Avillous

First view Second view Stereo view

  • 0.1 - 0.2 mm.
  • Solid synctio and cytotrophoblast without lacunar development.
  • Amniotic cavity.
  • A simple bilaminar germ disc.
  • Early mesoblast formation.
  • Ovulation age: 7 days.

QUESTION: True or False? In stage five the ovum is a complex bilaminar germ disc?

Horizon 5b

First view Stereo view Second view

  • Synctiotrophoblastic lacunae with early utero-placental circulation.
  • Amniogenesis.
  • A simple bilaminar germ disc.
  • Variable degrees of exocoelomic (Heuser's) membrane formation.
  • Age: 9 days.

QUESTION: What is the state of utero-placental circulation in this 5b stage embryo?

Horizon 5C

First view Second view Third view

  • Intercommunicating lacunar spaces filled with maternal plasma containing increasing numbers of blood cells.
  • A distended chronic cavity of maximum size.
  • A bilaminar germ disc with no axis.
  • A well defined amnion and amniotic cavity.
  • The endometrial epithelium is proliferating to repair the penetration defect. From this defect created by the ovum arises a coagulum which communicates with maternal blood within the lacunae.
  • Ovulation age: 11 days.

Download PDF

Developmental Stages in Human Embryos by Ronan O'Rahilly and Fabiola Müller.
Published by Carnegie Institution of Washington, Publication 637. 1987.

Stage 5 (PDF 686 KB)


QUESTION: What fills the intercommunicating lacunar spaces?