16.33 Human birth: day 12-28 Level 2 and 3 Life  
     
  13 days post-ovulation  
  0.2 mm  
 
 
  Chorionic villi "fingers" in the forming placenta now anchor the site to the uterus.
  
  The formation of blood and blood vessels of the embryo begins in this stage. The blood system appears first in the area of the "placenta" surrounding the embryo, while the yolk sac begins to produce hematopoietic or non-nucleated blood cells.
  
  By the end of this stage, the embryo is attached by a connecting stalk (which will later become part of the umbilical cord), to the developing placenta.
  
  The next stage begins when a narrow line of cells appears on the surface of the embryonic disc. This primitive streak is the future axis of the embryo and it marks the beginning of gastrulation, a process that gives rise to all three layers of the embryo: ectoderm, mesoderm and endoderm.
  
  16 days post-ovulation  
  0.4 mm  
 
 
  In this stage, gastrulation began with the appearance of the primitive streak. In the next stage, gastrulation continues with the formation of the audoderm and mesoderm, which develop from the primitive streak, changing the two-layered disc into a three-layered disc. The cells in the central part of the mesoderm release a chemical causing a dramatic change in the size of the cells in the top layer (ectoderm) of the flat disc-shaped embryo. The ectoderm grows rapidly over the next few days forming a thickened area. The three layers of the will eventually give rise to:
  
  Endoderm that will form the lining of lungs, tongue, tonsils, urethra and associated glands, bladder and digestive tract.
  
  Mesoderm that will form the muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, and reproductive and excretory systems.
  
  Ectoderm that will form the skin, nails, hair, lens of eye, lining of the internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands, and all parts of the nervous system.
  
  17-19 days post-ovulation  
  1.0 - 1.5 mm  
 
 
  The embryonic area is now shaped like a pear, and the head region is broader than the tail end.
  
  The ectoderm has thickened to form the neural plate. The edges of this plate rise and form a concave area known as the neural groove. This groove is the precursor of the embryo's nervous system and it is one of the first organs to develop.
  
  By the next stage, the blood cells of the embryo are already developed and they begin to form channels along the epithelial cells which form consecutively with the blood cells.
  
  19 - 21 days post-ovulation
  
  1.5 - 2.5 mm  
 
 
  By this stage, if you could look at the embryo from a top view, it would resemble the sole of a shoe with the head end wider than the tail end, and a slightly narrowed middle.
  
  Somites, which are condensations composed of mesoderm, appear on either side of the neural groove. The first pair of somites appear at the tail and progress to the middle. One to three pairs of somites are present by this stage.
  
  Every ridge, bump and recess now indicates cellular differentiation.A head fold rises on either side of the primitive streak. The primitive streak now runs between one-fourth to one-third of the length of the embryo.
  
  Secondary blood vessels now appear in the chorion/placenta. Hematopoietic cells appear on the yolk sac simultaneously with endothelial cells that will form blood vessels for the newly emerging blood cells.
 
  Endocardial (muscle) cells begin to fuse and form into the early embryo's two heart tubes.
 
  21 - 23 days post-ovulation
  
  1.5 - 3.0 mm
  
 
 
  Stage 10 reflects rapid growth and change as the embryo becomes longer and the yolk sac expands.
 
  On each side of the neural tube, between four and twelve pairs of somites can exist by the end of Stage 10. The cells which become the eyes appear as thickened circles just off of the neural folds. The cells of the ears are also present.
 
  Neural folds are rising and fusing at several points along the length of the neural tube concomitant with the budding somites which appear to "zipper" the neural tube closed. Neural crest cells will eventually contribute to the skull and face of the embryo.
  
  The two endocardial tubes formed in Stage 9 fuse in Stage 10 to form one single tube derived from the roof of the nueral tube, which becomes S-shaped and makes the primitive heart asymmetric. As the S-shape forms, cardiac muscle contraction begins.
  
  23 - 25 days post-ovulation
  
  2.5 - 3.0 mm  
 
 
  Thirteen to twenty pairs of somites are present in Stage 11 and the embryo is shaped in a modified S curve. The embryo has a bulb-like tail and a connecting stalk to the developing placenta.
  
  A primitive S-shaped tubal heart is beating and peristalsis, the rhythmic flow propelling fluids throughout the body, begins. However, this is not true circulation because blood vesel development is still incomplete.
  
  At this stage, the neural tube determines the form of the embryo. Although the primary blood vessels along the central nervous system are connecting in this stage, the central nervous system appears to be the most developed system. If twenty somites are present in the embryo, the forebrain is completely closed.  
  25 - 27 days post-ovulation
  
  3.0 - 5.0 mm  
 
 
  The embryo curves into a C shape. The arches that form the face and neck are now becoming evident under the enlarging forebrain. By the time the neural tube is closed, both the eye and ear will have begun to form. At this stage, the brain and spinal cord together are the largest and most compact tissue of the embryo.
 
  A blood system continues to develop. Blood cells follow the surface of yolk sac where they originate, move along the central nervous system, and move in the chorionic villi, the maternal blood system.
 
  Valves and septa may appear in the heart in this stage. The digestive epithelium layer begins to differentiate into the future locations of the liver, lung, stomach and pancreas. The beginning cells of the liver form before the rest of the digestive system.

  
     
 

 

  
 
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