Obgyn Key

As the notochord and neural tube form in the third week, the intraembryonic mesoderm lateral to these structures thickens to form two longitudinal columns of paraxial mesoderm ( Fig. 14.1…

The muscular system develops from mesoderm , except for the muscles of the iris of the eye, which develop from neuroectoderm (neural crest cells) , and the muscles of the…

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…

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…

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…

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…

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…

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…

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…

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…