Embryologic Development of Animals 453 



cavity and the trophectoderm is quite large. The yolk sac cavity con- 

 tains no yolk (food), but its upper region will later form the roof of the 

 alimentary canal. The amniotic and yolk sac cavities are separated by 

 a cellular embryonic disk (embryonic shield) which will form the em- 

 bryo. Its ventral layer is entodermal, its dorsal layer ectodermal, and 

 the cells between the two are mesodermal (Fig, 224). Mesoderm is also 

 formed between the yolk sac cavity and the trophectoderm and between 

 the ectodermal lining of the amniotic cavity and the trophectoderm. The 

 trophectoderm is now lined on the inside by a layer of mesoderm (Fig. 

 224), known specifically as somatic mesoderm. The trophectoderm and 

 somatic mesoderm combined are known as the chorion because the lat- 

 ter, through minute projections (villi), contacts the blood vessels of the 

 uterus in order to supply nourishment until the future blood system of 

 the embryo is developed. Hence, the entoderm of the yolk sac cavity 

 is covered with splanchnic mesoderm, while the ectoderm of the amniotic 

 cavity is covered with somatic mesoderm (Fig. 224) . The blastocyst also 

 develops a third cavity, the extraembryonic coelom, located between the 

 two separating layers of mesoderm. These two layers were originally 

 one layer which was located between the entoderm of the yolk sac cavity 

 and the trophectoderm. 



In the next stages of development the embryonic mass (embryo) 

 detaches itself partially from the inner surface of the chorion, grows rap- 

 idly, and forms a tubular outgrowth from the upper region of the yolk 

 sac (Fig. 224) . This outgrowth, called the allantois (Gr. allanto, sausage 

 or tubular; eidos, form), grows toward the chorion and through the body 

 stalk, by means of which the embryo is attached to the chorion. The 

 yolk sac and allantois do not play as great a role in human embryologic 

 development as they do in lower forms of organisms. 



Between the third and fourth week of gestation (L. gestatio, carry- 

 ing, pregnant) the blastocyst has enlarged so as to form a bulge on the 

 surface of the uterus. The embryo soon pushes into the cavity of the 

 uterus, being surrounded by the amnion membrane of the embryo (Gr. 

 amnion, embryo covering) . The body stalk now functions as the um- 

 bilical cord, the latter being continuous with the highly vascular, disk- 

 shaped placenta which in turn is in contact with the blood vessels of the 

 walls of the uterus. 



About two- thirds of the embryonic shield (embryonic disk) previously 

 mentioned will form the future head, while the remainder will form the 

 neck, trunk, and tail. The two cellular layers of the embryonic shield 

 consist of (1) the lower entoderm layer (nearest the yolk sac cavity) and 



