The lower respiratory organs (larynx, trachea, bronchi, and lungs) begin to form during the fourth week of development. The respiratory system starts as a median outgrowth—the laryngotracheal groove —that appears in the floor of the caudal end of the primordial…
The pharyngeal apparatus consists of pharyngeal arches, pharyngeal pouches, pharyngeal grooves, and pharyngeal membranes ( Fig. 10.1 ). These embryonic structures contribute to the formation of the face and neck. Open full size image Fig. 10.1 Illustrations of the pharyngeal…
Early in the fourth week of development, the intraembryonic coelom —the primordium of the body cavities—appears as a horseshoe-shaped cavity ( Fig. 9.1 A ). The bend in this cavity at the cranial end of the embryo represents the future…
The fetal part of the placenta and fetal membranes separate the embryo or fetus from the endometrium —the inner layer of the uterine wall. The chorion, amnion, umbilical vesicle, and allantois constitute the fetal membranes. An interchange of substances (e.g.,…
Development during the fetal period is concerned primarily with body growth and differentiation of tissues, organs, and systems. Rudimentary organ systems were formed during the embryonic period. The rate of body growth during the fetal period is rapid, and fetal…
All major external and internal structures are established during the fourth to eighth weeks. By the end of this period, the main organ systems have begun to develop. Exposure of embryos to teratogens (e.g., drugs and viruses) during this period…
Rapid development of the embryo from the trilaminar embryonic disc during the third week is characterized by: Appearance of the primitive streak Development of the notochord Differentiation of three germ layers The third week of development occurs during the week…
Implantation of the blastocyst is completed during the second week of development. As this process takes place, changes occur, producing a bilaminar embryonic disc composed of two layers, the epiblast and hypoblast ( Fig. 4.1 A ). The embryonic disc…
Development begins at fertilization when a sperm penetrates an oocyte to form a zygote. A zygote is a highly specialized, totipotent cell, which has the ability to differentiate into any type of cell. It contains chromosomes and genes derived from…
Puberty begins when secondary sex characteristics appear, usually between the ages of 10 to 13 years in females and 12 to 14 years in males. Menarche (first menstrual period) may occur as early as 8 years. Puberty in females is…