| 1 (HH1) |
1–12 |
The area pellucida was easily distinguished from the area opaca; small clusters of cells emerged and formed a reticular structure; hypoblasts formed; the marginal zone, the thick area at posterior zone, became prominant between the two-layered germinal discs. |
| 2 (HH2) |
10–14 |
Initial streak formation: the primitive streak initiated as a short conical condensation in the area pellucida (0.34–0.61 mm in length); margins of the marginal zone and the primitive streak were not clearly separated yet. |
| 3 (HH3) |
13–24 |
Elongation of the intermediate or growing streak: the primitive streak extended to the center of area pellucida and became a dense bulge 19 h post-incubation. The primitive streak was separated from the margin of the marginal zone and became a rostral-caudal mid-line within the margin of the blastoderm, where the primitive groove was still not clearly visible (0.39–1.07 mm in length). |
| 4 (HH4) |
25–32 |
Formation of the definite or full-length streak: the primitive streak reached its maximal length (0.73–1.25 mm, average length = 0.93 mm). The primitive pit, primitive groove, and Hensen’s node were prominent with no head process visible yet. The primitive streak ultimately extended about 2 to 3 times of its initial length. The area pellucida became somewhat V-shaped by 28 h post-incubation. |
| 5 (HH5) |
25–35 |
The head process became visible but no headfolds formed yet by 34 h post-incubation. The notochord extended from the head position caudally to Hensen’s node. The area pellucida became elongated along the anterio-posterial axis. The initial neurulation; the neural plate became clearly visible by the end of this period. |
| 6 (HH6) |
31–36 |
Headfolds appeared by 31 h of incubation, but no somite formation was identified yet. Hensen’s node shifted to the central area of the embryo between the notochord and the primitive streak. |
| 7 (HH7) |
32–34 |
Initiation of somitogenesis; one to 3 pairs of somites were formed by 33 h of incubation and neural folds appeared on both sides of the neural plate in the cephalic region. The notochord prolonged and the Hensen’s node started to migrate caudally accompanied with the shortening of the primitive streak. |
| 8 (HH8) |
32–36 |
The 4th to 5th pair somite stage; the neural folds curved dorsally and met around the midbrain. The subcephalic pocket became visible 36 h post-incubation. |
| 9 (HH9) |
36–42 |
The 6th to 9th pair somite stage; the primary optical vesicles were first recognized by 42 h of incubation, and paired heart primordia started to fuse. The neural groove closed and formed the neural tube accompanied by the increasing size of the subcephalic pocket 41 h after incubation. |
| 10 (HH10) |
43–46 |
The 10th to 11th pair somite stage; the heart loop slightly bent to the left and the three primary brain vesicles were first visible by 43 h after incubation. The omphalomesenteric veins began to grow from the caudal end of the heart by 46 h of incubation. |
| 11 (HH11) |
46–52 |
The 12th to 14th pair somite stage; five neuromeres of the hindbrain were visible and the neuropore was not closed yet. The optic vesicles constricted at the base by 48 h of incubation and the heart was fully bent toward the left by 52 h of incubation. |
| 12 (HH12) |
52–58 |
The 16th to 17th pair somite stage; the neuropore was closed and the enlargement of proencephalon (the anterior part of the head) occurred; the primary optic vesicles and optic stalks were formed. The auditory pit was wide open by 57 h; the heart developed into slightly S-shaped by 58 h, and the head fold partially covered the forebrain. |
| 13 (HH13) |
54–60 |
The 18th to 19th pair somite stage; the head was turning to the left and the telencephalon was distinctly enlarged; the head fold from the amnion gradually covered the forebrain, midbrain and the anterior part of hindbrain. |
| 14 (HH14) |
59–63 |
The 21th to 23th pair somite stage; cranial and trunk flexures of embryos developed; By 63 h post incubation, opening of the auditory pit was defined; the amnion was extending close to somites 7 to 10. |
| 15 (HH15) |
62–65 |
The 24th to 25th pair somite stage; the head flexure was progressively evident; the optic cup was completely formed and the amnion extended from somites 7 to 15. |
| 16 (HH16) |
64–72 |
The 26th to 32th pair somite stage; the amnion grew and extended to somites 10 to 18; the primordia wing bud formed as a fine crest, whereas the leg bud was still invisible; tail bud appeared as a short straight cone; epiphysis was not formed yet. |
| 17 (HH17) |
71–72 |
The 32th to 34th pair somite stage; the head and body flexures were pronounced; the leg buds is lifted off and appeared as fine crest; tail bud appeared as a small bulge and bent somewhat ventrally. The amnion appeared but the allantois was not yet formed. |
