The morphologic alterations during the fifth week are subtle. The growth of the head predominates.
During this week the embryo measures between 2.5 and 7 mm. On its dorsal side 30 or more somites can be seen through the ectoderm.
The enlargement of the rostral poles is related to the rapid development of the nervous system. Thereby, the derivatives of the prosencephalon develop especially rapidly. The development of the face also contributes to the enlargement of the rostral pole. This growth leads to a more and more strongly pronounced mesencephalic, cervical and pontine flexures, until the head finally comes to lie on the umbilical cord. 14
The rostral neuropore closes at around the 29th day 11. The caudal neuropore closes only at around the 30th day 12.
In stage 13 the prosencephalon (forebrain) has divided itself into two sections:
The paired telencephalon from which the paired cerebral vesicles are generated.
The unpaired diencephalon, which lies further rostrally.
The anlagen of the upper extremities appear 12. Somewhat later also follow those of the lower extremities13.
The sensory organs also arise in the fifth week. The ophthalmic vesicle and the optic cup11, 14 develop, followed by the lens placode and the lens vesicle13, 14, as well as the nasal placode11, the auditory pit11 and finally the auditory vesicle13. Due to their pigmentation the lens placodes are more visible in stage 15 15 .
As was mentioned above, the growth of the central nervous system (especially the telencephalon) results in a flexure of the head (mesencephalon, cervical and pontine flexure). Only with the development of the future spinal cord does the embryo straighten itself out again.
During the fifth week the presence of an embryonic tail is characteristic; this atrophies again, though, during the 6th to 8th weeks.
In stage 14 14 the cardiac prominence is clearly visible.
Development of the pharyngeal arches
(stages 11 -14)
At the beginning of the fifth week the third and fourth pharyngeal arches develop. 12, 13
In stage 14 14, the pharyngeal arches are located directly above the heart. The second pharyngeal arch (hyoid arch) begins to grow over the 3rd and 4th pharyngeal arches and forms the operculum and, behind it, a lateral, ectodermal pit: the sinus cervicalis (visible from stage 12).
Fig. 16 - The sinus cervicalis at approximately 33 days (stage 14)