Animal development Chapter 37

Direct and Indirect Development People and other mammals – as well as birds and reptiles – are all born as “miniature adults” developing via a process called direct development. In indirect development, a juvenile animal differs significantly from an adult of the same species and undergoes radical changes in body form during development, such as the transformation of a caterpillar into a butterfly.

Indirect Development Butterfly

Main Ideas The fertilized egg implants into the uterus and is nourished by the placenta. The zygote develops into a fully formed fetus in about 38 weeks The mother affects the fetus, and pregnancy affects the mother.

Connect to your world A human zygote develops from a single cell into a fully formed human in about nine months. The rate of growth in the first few weeks is astonishing, if you grew at the same rate after birth, you would be 4 meters (13ft) tall at one month of age. The zygote’s growth is directed by its DNA. However, the environment of the uterus and the mother’s overall health also have a strong impact on how well the zygote develops.

Cleavage A human egg is usually fertilized in the mother’s oviduct and undergoes cleavage divisions on its way to the uterus , a journey that takes about 4 days. By about 1 week after fertilization, the zygote has developed into a hollow ball of cells, known as the blastocyst. The blastocyst consists of a hollow ball of cells with a thicker inner cells mass, and the sticky outer wall will adhere to the uterus and burrow into the endometrium, a process called implantation.

The Journey of the Egg

Blastocyst The outer layer of the blastocyst will first become the chorion and will later form the embryonic contribution to the placenta. The inner cell mass develops into the embryo and the three other extra embryonic membranes.

Gastrulation A stage in which three cell layers develop from the balstocyst: the ectoderm, the mesoderm, and the endoderm. The ectoderm layer develops into the skin and nervous system. The mesoderm layer forms many of the internal tissues and organs. The endoderm layer develops into many of the digestive organs and the lining of the digestive system. Once these structures begin to form, the ball of cells is known as an embryo.

Development of Fetus Fertilization Zygote Cleavage Gastrulation Organogenesis Growth Birth

Embryonic Membranes Amnion Chorion Placenta

Role of placenta in development Placenta connects the mother and embryo to allow for the exchange of oxygen, nutrients, and wastes between them. Production of hormones – Human chorionic gonadotropin, estrogen and progesterone.

Umbilical cord It is the connecting cord from the developing embryo or fetus to the placenta. It actually consists of two arteries and veins that are twisted together.

Birth During the last month of pregnancy, the fetus becomes positioned head downward in the uterus, with the crown of the skull resting against the cervix. Normally the process of birth begins around the end of the ninth month. Birth results from a complex interplay between uterine stretching caused by growing fetus and fetal and maternal hormones that trigger labor.

Just before birth

Stages involved in birth process 1. Dilation of the cervix -hormones released by mother and fetus Cervix dilates at least 10 centimeters. If it does not dilate the doctor will follow cesarean section or c-section. Emergence of the baby: powerful contraction of the uterus help rotate the baby so that its head is toward the cervix. Head emerges, followed by the rest of the body. Within a short time, the baby is breathing. Expulsion of the placenta:

Cesarean section

Structure of mammary glands Colostrum: during the first few days after birth, the mammary glands secrete a thin, yellowish fluid called colostrum. Colostrum is high in protein and contains antibodies that help protect the new born against some diseases and are absorbed directly through the infant’s intestine. It is gradually replaced by mature milk which is higher in fat and milk sugar (lactose )and lower in protein.

Twin babies

Test tube babies