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Figure 61. Early stages of the human embryo. 



A. Mature ovum, after Streeter, 48 about 1 millimeter in diameter, view from above, 

 with the amniotic roof removed to show the early embryonic disk and yolk sac. ps, 

 primitive streak; cs, connecting stalk. 



B. Diagram of the human embryonic disk at a slightly later stage, viewed from above, 

 to show the various organ-forming germinal areas. From Jackson. 23 ng, neural 

 groove; ps, primitive streak; pp, primitive pit; pn, primitive node; U, upper limb; 

 L, lower limb. 



C. Embryo 2.11 millimeters long, after Eternod, viewed from above, amnion removed. 

 am, cut edge of amnion; nt, neural tube (closed); nt' , nt", neural tube still an 

 open groove in brain region anteriorly and spinal cord region posteriorly; s, meso- 

 dermic somite, or body segment; pg, primitive groove (streak) and tail bud region. 



