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Figure 24. 12 Gastrulation - Formation of the three germ layers

As the blastocyst reaches the uterus, it attaches and sinks in the uterine endometrium. This is called implantation and occurs 7-8 days following fertilization. During implantation, the trophoblast cells secrete enzymes which help the blastocyst to attach and implant. The normal site of implantation is in the posterior wall of the uterus. Implantation helps the blastocyst to absorb nutrients from the glands and blood vessels of the endometrium for its growth and development.


Gastrulation is defined as a dynamic process during which rearrangement and reorganization of the cells take place to form the three primary germ layers: the ectoderm, endoderm and the mesoderm.

In humans, the formation of the germ layers happens so quickly that it is difficult to determine the exact sequence of events.

(1) Before implantation, a layer of ectoderm (the trophoblast layer) has been formed around the blastocoel (Figure 24.12A). The trophoblast cells will become part of the chorion #one of the membranes surrounding the fetus.

(2) The inner cell mass moves downwards, and a space called the amniotic cavity is formed within the inner cell mass. The bottom layer of the inner cell mass develops into an endodermal germ layer (Figure 24.12B)

(3) Now a layer of cells grows around the top of the amniotic cavity, to form the amnion-another fetal membrane. The cells below the cavity form the embryonic disc, from which the embryo develops. The embryonic disc contains scattered ecto, meso and endodermal cells.

(4) The endodermal cells now divide rapidly and extend downward in a circle (Figure 24.12C). In birds and reptiles this circle is the yolk-sac yet another fetal membrane. In mammals, an allantoic membrane also forms from endosperm, which stores metabolic wastes in some species. Some of the mesodermal cells also move into the structures of the fetal membrane.

(5) The embryonic disc separates into 3 distinct layers: The upper ectoderm, the middle mesoderm, and the lower endoderm (Figure 24.12D). The embryonic disc gradually squeezes off the yolk-sac, and the cavity inside the disc is the endoderm-lined primitive gut. The mesoderm within the disc soon splits into an outer somatic and inner splanchnic mesoderm. The space between these layers becomes the coelom, or body cavity.

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Table of Contents

24.0 Introduction
24.1 Human reproduction
24.2 Female reproductive cycle
24.3 Embryonic Development

Chapter 25


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