274 EMBRYOLOGY 



The human egg is liberated into the opening of the oviduct about 14 days 

 after the beginning of the last period of menstruation. Fertilization takes 

 place in the upper part of the oviduct, and a period of cleavage ensues for 

 about three days. At about 6 days after fertilization the blastocyst has pre- 

 sumably formed and a large cavity (the blastocoel?) becomes filled with a 

 fluid. There is probably an aggregation of cells at one pole of the blastocyst, 

 known as the inner cell mass (Fig. 170). At this time the blastocyst floats 

 freely within the uterine cavity. At about iy 2 days the blastocyst has become 

 implanted in the wall of the uterus. The amniotic cavity appears, and the 

 endoderm begins to segregate from the inner cell mass, as in Figure 171. In 

 the next few days the yolk sac forms and becomes encased in mesoderm. At 

 14 to 16 days a primitive streak develops (Fig. 173) . During the l4-to-2 1-day 

 period the neural plate forms and folds into a tube, the heart develops, and 

 the digestive tract shows some differentiation. At this point we shall proceed 

 to a more detailed discussion of the structure and function of the human 

 embryo. 



Human embryo at three weeks 



The embryo shown in Figure 184 is about three weeks old, has ten 

 somites, and is about 2 mm. long. By this time the neural tube has closed 

 throughout most of its length. It remains open anteriorly and posteriorly. 

 There is relatively little differentiation within the brain region, although the 

 three primary divisions are roughly delineated. A thickening of the ectoderm 

 opposite the rhombencephalon is the otic disk, or auditory placode, which 

 will form the internal ear. 



In the anterior brain the optic primordia are present, and on either side 

 of the neural tube the neural crests have formed. In this embryo a notochord 

 runs the length of the somites; it is still fused with the underlying endoderm 

 and remains in contact with the neural tube above. 



The endoderm at this time is divided into a foregut, midgut, and hindgut. 

 The primordia for the thyroid and the laryngotracheal groove are present in 

 the foregut. From the hindgut the allantois forms an outgrowth running 

 posteriorly into the body stalk. The umbilical veins and arteries follow this 

 allantois into the placenta. 



The mesoderm shows division into the somites, the rudiment of the pro- 

 nephros, and the lateral mesoderm, which forms two layers, enclosing a 



