title: Basic Concepts in Embryology: A Student's Survival Guide
author:  Lauren J. Sweeney 
description: McGraw-Hill authors represent the leading experts in their fields and are dedicated to improving the lives, careers, and interests of readers worldwide
zlib_text: 180
  During fertilization, the acrosome of a sperm releases en-
  zymes that digest a hole in the jelly coat around the egg
  and then extrudes a filament that attaches to a receptor
  on the vitelline membrane. The sperm nucleus enters the
  egg and fuses with the egg nucleus.
  During the early developmental stages, cleavage
  leads to a morula, which becomes the blastula when an
  internal cavity (the blastocoel) appears. Then, at the gas-
  trula stage, invagination of cells into the blastocoel re-
  sults in formation of the germ layers: ectoderm,
  mesoderm, and endoderm. During neurulation, the ner-
  vous system develops from midline ectoderm, just above
  the notochord. At this point, it is possible to draw a typ-
  ical cross section of a vertebrate embryo.
  Differentiation begins with cleavage, when the egg’s
  cytoplasm is partitioned among the numerous cells. In a
  frog embryo, only a daughter cell that receives a portion
  of the gray crescent is able to develop into a complete em-
  bryo. Morphogenesis in