cavities are populated by hematopoietic stem cells (answer c)beginning in
the second month of fetal life. The connective tissue capsule and skeletal
framework of the spleen develop from splanchnic lateral plate mesoderm dur-
ing the fifth week and are quickly invaded by hematopoietic cells of the
myeloid lineage (answer d).It remains an active hematopoietic organ until at
least the seventh month in utero. Blood islands develop by differentiation of
mesodermal cells in the extraembryonic mesoderm lining the yolk sac during
the third week of fetal development (answer e).They give rise to vitelline ves-
sels and are the major site of red blood cell formation in the early embryo.
17.The answer is c.(Moore and Persaud, Developing, pp 78, 80–81. Sadler,
pp 79–80, 159–162.)Cranial folding is responsible for the placement of the
developing heart in the presumptive thoracic region of the embryo. Ini-
tially, the developing cranial portion of the neural tube lies dorsal and cau-
dal to the oropharyngeal membrane. However, overgrowth of the forebrain
causes it to extend past the oropharyngeal membrane and overhang the
cardiogenic area. Subsequent growth of the forebrain pushes the develop-
ing heart ventrally and caudally to a position in the presumptive thoracic
region caudal to the oropharyngeal membrane and cranial to the septum
transversum that will form the central tendon of the diaphragm. Gastrula-
tion(answer a)is the process by which epiblast cells migrate to the prim-
itive streak and become internalized to form the mesodermal and
endodermal germ layers. Lateral folding (answer b)of the embryo forms
the endoderm tube and surrounding concentric layering of mesoderm and
ectoderm. Neurulation refers to formation of the neural tube from surface
ectoderm(answer d).The fusion of the two endocardial heart tubes
(answer e)occurs as lateral folding occurs. The fused tube will form the
endocardium surrounded by the primordial myocardium derived from
splanchnic mesoderm that will form the heart muscle (myocardium).
18.The answer is d.(Moore and Persaud, Developing, pp 45–49,126. Sadler,
pp 45–49.)In the developing fetus, the maternal blood is in direct contact with
the syncytiotrophoblast. During implantation, the syncytiotrophoblast invades
the endometrium and erodes the maternal blood vessels. Maternal blood and
nutrient glandular secretions fill the lacunae and bathe the projections of syn-
cytiotrophoblast. Primary villi consist of syncytiotrophoblast with a core of
cytotrophoblast cells. In secondary villi, the cytotrophoblast core is invaded by
mesoderm and subsequently by umbilical blood vessels in tertiary villi.
Embryology: Early and General Answers 89