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Introduction

The embryonic stage of fetal development, which occurs 3-8 weeks after fertilization (ie, weeks 5-10 of gestation), involves the differentiation of specialized tissues and organs from a single fertilized egg.  Key to this process are gastrulation and neurulation, which lead to the formation of the 3 primary germ layers—ectoderm, mesoderm, and endoderm—and the subsequent development of the central nervous system.

Gastrulation

Gastrulation ( Figure 1) marks the beginning of the embryonic stage of development.  It is the process by which the blastocyst, a hollow ball of cells, reorganizes from a bilaminar disc into a trilaminar (3-layered) disc (ectoderm, mesoderm, endoderm), from which all tissues and organs are derived.  This process occurs during week 3 postfertilization and begins with the formation of the

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