T he alimentary system (digestive system) is the digestive tract from the mouth to the anus, with all its associated glands and organs. The primordial gut forms during the fourth week as the head, caudal eminence (tail), and lateral folds…
The lower respiratory organs (larynx, trachea, bronchi, and lungs) begin to form during the fourth week of development. Respiratory Primordium 9 The respiratory system starts as a median outgrowth, the laryngotracheal groove , which appears in the floor of the…
T he pharyngeal apparatus consists of pharyngeal arches, pouches, grooves, and membranes ( Fig. 9.1 ). These early embryonic structures contribute to the formation of the face and neck. Open full size image Fig. 9.1 Pharyngeal apparatus. A , Dorsal…
Early in the fourth week of development of the embryo, the intraembryonic coelom appears as a horseshoe-shaped cavity ( Fig. 8.1 A ). The bend in the cavity at the cranial end of the embryo represents the future pericardial cavity…
The placenta and fetal membranes separate the fetus from the endometrium , the inner layer of the uterine wall. An interchange of substances, such as nutrients and oxygen, occurs between the maternal and fetal bloodstreams through the placenta. The vessels…
The transformation of an embryo to a fetus is gradual, but the name change is meaningful because it signifies that the primordia of all major systems have formed. Development during the fetal period is primarily concerned with rapid body growth…
All major external and internal structures are established during the fourth to eighth weeks. By the end of this embryonic period, the main organ systems have started to develop. As the tissues and organs form, the shape of the embryo…
Rapid development of the embryo from the trilaminar embryonic disc during the third week (see Fig. 4.3 H ) is characterized by: Appearance of primitive streak Development of notochord Differentiation of three germ layers The third week of development coincides…
As implantation of the blastocyst occurs, morphologic changes in the embryoblast produce a bilaminar embryonic disc composed of epiblast and hypoblast ( Fig. 3.1 A ). The embryonic disc gives rise to the germ layers that form all the tissues…
Human development begins at fertilization when a sperm fuses with an oocyte to form a single cell, the zygote . This highly specialized, totipotent cell (capable of giving rise to any cell type) marks the beginning of each of us…