8.4 The most important events in the third to eighth developmental weeks

• Introduction to the development processes during the third week (stages 6 to 7; ca. 17-19 days)
• Fourth week (stages 8 to 10; ca. 23-28 days)
• Fifth week (stages 11 to 14; ca. 29-33 days)
  
  • development of the pharyngeal arches
• Sixth week (stages 15 to 17; ca. 36-41 days)
  • development of the pharyngeal arches
• Seventh week (stages 18 to 20; ca. 44-49 days)
• Eighth week (stages 21 to 23; ca. 51-56 days)

Sixth week
(stages 15 to 17; ca. 36-41 days)

In the course of the sixth week, close to each of the first two pharyngeal arches, three auricular hillocks develop 15. They delimit the external auditory canal 17.

The mesonephrogenic prominence 16 forms and indicates the location of the future uro-genital system.

During the sixth week a small intestinal loop invades the extraembryonic coelom and into the proximal section of the umbilical cord, because the abdominal cavity is still too small to contain all of it. A physiologic umbilical hernia (stage 16) occurs. This is a normal phenomenon that is due to the rapid growth of the intestines.

The cranial 17 and spinal ganglia arise from neural crest cells 9 .

The cerebral vesicle 14 bends forwards and downwards. The rostral pole of the embryo rests on the cardiac prominence.

The liver makes its appearance between the pericardium region and the umbilical cord as the liver prominence.

The differentiation of the arms and legs progresses rapidly, whereby the upper extremities are always somewhat further developed than the lower ones. One can already see the interdigital zones 17.

The chondrogenesis of the future bones begins in the condensed mesenchymal blastemas (chondrific centers).

Development of the pharyngeal arches
(stage 15 -17)

In the course of the sixth week the lower edge of the 2nd pharyngeal arch covers the 3rd and 4th arches, which shift towards the rear and for now form the base of the sinus cervicalis that is closing itself.