5.1 The cleavage divisions and the migration of the embryo through the tube

• The cleavage divisions up to the morula stage
  
• How a blastocyst is engendered
• The emergence of the blastocyst (hatching)
• The polarity of the embryo
• The migration of the embryo through the fallopian tube

The emergence of the blastocyst (hatching)

Around the end of the fifth day the embryo frees itself from the enveloping pellucid zone. Through a series of expansion-contraction cycles the embryo bursts the covering. This is supported by enzymes that dissolve the pellucid zone at the abembryonic pole. The rhythmic expansions and contractions result in the embryo bulging out of and emerging from the rigid envelope. This "first birth" is called hatching.

Fig. 11, Fig. 12
The embryo emerges from the pellucid zone, leading with the abembryonic pole first.

Fig. 13
Characteristic for the volume increase are the rhythmic expansion and contractions as seen in the figure.
After M.P. Primi, CHUV, Lausanne