Ch 29

Fertilized egg 

Once the dividing embryo consists of 12 or more cells, it is a solid sphere composed of numerous, smaller spheres and is called a morula

 Four or 5 days after ovulation, the morula consists of about 32 cells. Near this time, a fluid-filled cavity called theblastocele (blas′tō-sēl) begins to form near the center of the cellular mass, resulting in a hollow sphere called a blastocyst

process when the blastocyst implants into the wall of the uterus between day 5and 6 by secreting an enzyme which wears down the epithelial lining

The embryonic period comprises days 14–56 after fertilization (weeks 3–8 of development) and is the time when the major organ systems form. The developing human is called an embryo (em′brē-ō) at this stage.

The fetal period extends from 56 days after fertilization to birth (the last 30 weeks of development). During this stage, the organ systems grow and become more mature, and the developing human is called a fetus (fē′tus).

After implantation, an amniotic (am-nē-ot′ik) cavity forms inside the inner cell mass and is surrounded by a layer of cells called the amnion

Branches, called chorionic (kō-rē-on′ik) villi, sprout from the cords of cytotrophoblasts and protrude into the lacunae, like fingers. The entire embryonic structure facing the maternal tissues is called the chorion

A third cavity, the yolk sac, forms inside the blastocele from the hypoblast

Before birth, the fetal blood passes to the placenta through umbilical arteries from the internal iliac arteries and returns through an umbilical vein.At birth, when the umbilical cord is tied and cut, no more blood flows through the umbilical vein and arteries, and they degenerate.The remnant of the umbilical vein becomes the ligamentum teres, or round ligament, of the liver, and the ductus venosus becomes the ligamentum venosum.The remnants of the umbilical arteries become the cords of the umbilical arteries.

 blood flows from the right atrium, through the foramen ovale, and into the left atrium.

The blood passes through the liver via the ductus venosus, which joins the inferior vena cava.The remnant of the umbilical vein becomes the ligamentum teres, or round ligament, of the liver, and the ductus venosus becomes the ligamentum venosum.

Within 1 or 2 days after birth, the ductus arteriosus, which connects the pulmonary trunk to the aorta and allows blood to flow from the pulmonary trunk to the systemic circulation, closes off.Once closed, the ductus arteriosus is replaced by connective tissue and is known as the ligamentum arteriosum.If the ductus arteriosus does not close completely, it is said to be patent.This is a serious birth defect, resulting in marked elevation in pulmonary blood pressure because blood flows from the left ventricle to the aorta, through the ductusarteriosus to the pulmonary arteries.

me of these cells migrate toward and displace the hypoblast to form the endoderm (en′dō-derm; inside layer). Other cells emerge between the epiblast and the endoderm as the mesoderm(mez′o-derm; middle layer; figure 29.6). Those epiblast cells that do not migrate form the ectoderm (ek′tō-derm; outside layer). These three germ layers, the endoderm, mesoderm, and ectoderm, are the beginning of the embryo proper.

 If the blastocyst implants near the cervix, a condition called placenta previa (prē′vē-ă) occurs. As the placenta grows, it may extend partially or completely across the internal cervical opening. As the fetus and placenta continue to grow and the uterus stretches, the region of the placenta over the cervical opening may tear, and hemorrhaging may occur

abruptio (ab-rŭp′shē-ō)placentae, occurs when a normally positioned placenta tears away from the uterine wall. This condition is also accompanied by hemorrhaging.

involves the movement of epiblast cells and results in the formation of three distinct germ layers that eventually give rise to the many body structures.

A cordlike structure called the notochord extends from the cephalic end of the primitive streak

The lateral edges of the neural plate begin to rise, like two ocean waves coming together. These edges are called the neural folds, and between them lies a neural groove (figure 29.7). The underlying notochord stimulates the folding of the neural plate at the neural groove. The crests of the neural folds begin to meet in the midline and fuse into a neural tube.

The period between 14 and 60 days 

The cloaca is associated with 2 tubes. The hind gut and the allantois, a blind tube extending into the umbilical cord. The part of the allantois nearest the cloaca enlarges to form the urinary bladder

As the mesonephros is developing, the caudal end of the hindgut begins to enlarge to form the cloaca (klō-ā′kă; sewer), the common junction of the digestive, urinary, and genital systems

Fine soft hair that covers the fetus. 

digestive Changes Meconium is a mixture of cells from the digestive tract, amniotic fluid, bile, and mucus excreted by the newborn.

The urinary system develops in three stages—pronephros, mesonephros, and metanephros—from the head to the tail of the embryo. As the mesonephros is developing, the caudal end of the hindgut begins to enlarge to form the cloaca (klō-ā′kă; sewer), the common junction of the digestive, urinary, and genital systems (figure 29.Its distal end enlarges and branches to form the duct system of the adult kidney, called the metanephros (last kidney), which takes over the function of the degenerating mesonephros. 

In addition, the placenta secretes a growth hormonelike substance (human somatotropin) and a prolactin-like substance (human placental lactogen), both of which support the development of the breasts.Prolactin, produced by the anterior pituitary gland, is responsible for milk production.

Breast feeding stimulates the release of oxytocin from the mother's posterior pituitary gland, which is responsible for milk letdown.Oxytocin can also cause uterine contractions and cramps.