Ectoderm is one of three germ layers--groups of cells that coalesce early during the embryonic life of all animals except maybe sponges, and from which organs and tissues form. As an embryo develops, a single fertilized cell progresses through multiple rounds of cell division. Eventually, the clump of cells goes through a stage called gastrulation, during which the embryo reorganizes itself into the three germ layers: endoderm, ectoderm, and mesoderm. After gastrulation, the embryo goes through a process called neurulation, which starts the development of nervous system.

Possums is a 174-page book consisting of a series of essays written about the Virginia opossum (Didelphis virginiana), the only living marsupial in the US. The essays were written by Carl Gottfried Hartman, an embryologist at the Carnegie Institute of Washington (CIW), in Baltimore, Maryland, who also worked with another mammal, the rhesus monkey. Possums was published in 1952 by Hartman's alma mater, the University of Texas at Austin (UT). Beginning in 1913, while as a graduate student, and later as an instructor at UT, Hartman captured and raised opossums. He was one of the first zoologists to study the intricacies of opossum embryology, leading to an account of the embryology and reproductive physiology of a mammal from the wild, rather than of a mammal bred exclusively for laboratory research. Possums culminated Hartman's studies of the marsupial.

Implantation is a process in which a developing embryo, moving as a blastocyst through a uterus, makes contact with the uterine wall and remains attached to it until birth. The lining of the uterus (endometrium) prepares for the developing blastocyst to attach to it via many internal changes. Without these changes implantation will not occur, and the embryo sloughs off during menstruation. Such implantation is unique to mammals, but not all mammals exhibit it. Furthermore, of those mammals that exhibit implantation, the process differs in many respects between those mammals in which the females have estrous cycles, and those mammals in which the femals have menstrual cycles. Females in the different species of primates, including humans, have menstrual cycles, and thus similar processes of implantation.

To study human evolution, researchers sometimes use microstructures found in human teeth and their knowledge of the processes by which those structures grow. Human fetusus begin to develop teeth in utero. As teeth grow, they form a hard outer substance, called enamel, through a process called amelogenesis. During amelogenesis, incremental layers of enamel form in a Circadian rhythm. This rhythmic deposition leaves the enamel with microstructures, called cross-striations and striae of Retzius, which have a regular periodicity. Because enamel is not renewed throughout life like other tissues, teeth preserve the timing and details of a person's growth and development. Thus, enamel microstructures, from living people and from fossilized teeth, can be used to reconstruct the growth, development, and life histories of current and past humans. Researchers can also compare current and fossilized microstructures to trace changes in those traits over the course of human evolution.

The Notch signaling pathway is a mechanism in animals by which adjacent cells communicate with each other, conveying spatial information and genetic instructions for the animal's development. All multicellular animals utilize Notch signaling, which contributes to the formation, growth, and development of embryos (embryogenesis). Notch signaling also contributes to the differentiation of embryonic cells into various types of cells into various types of cells, such as neurons. Research into the Notch gene in fruit flies began in the early twentieth century, but not until the end of the twentieth century did researchers begin to uncover, in many different species, the roles of Notch proteins for cell to cell signaling. Researchers have also found that dysfunction in the pathway can contribute to diseases such as cancer and Alzheimer's.

In 1969, Roy J. Britten and Eric H. Davidson published Gene Regulation for Higher Cells: A Theory, in Science. A Theory proposes a minimal model of gene regulation, in which various types of genes interact to control the differentiation of cells through differential gene expression. Britten worked at the Carnegie Institute of Washington in Washington, D.C., while Davidson worked at the California Institute of Technology in Pasadena, California. Their paper was an early theoretical and mechanistic description of gene regulation in higher organisms.

David Starr Jordan studied fish and promoted eugenics in the US during the late nineteenth and early twentieth centuries. In his work, he embraced Charles Darwin s theory of evolution and described the importance of embryology in tracing phylogenic relationships. In 1891, he became the president of Stanford University in Stanford, California. Jordan condemned war and promoted conservationist causes for the California wilderness, and he advocated for the eugenic sterilization of thousands of Americans. Like many American eugenicists of the early twentieth century, Jordan combined ideas of Mendelian genetics and of Darwinian natural selection to form a basis for limiting or encouraging reproduction in certain individuals and groups based on their perceived hereditary fitness. Like other eugenicists, Jordan s attempt to control the reproductive fate of entire populations marked an episode in the history of reproduction and biology in which its concepts increasingly influenced the social and cultural contexts.

Dell Publishing in New York City, New York, published Lennart Nilsson's A Child Is Born in 1966. The book was a translation of the Swedish version called Ett barn blir till, published in 1965. It sold over a million copies in its first edition, and has translations in twelve languages. Nilsson, a photojournalist, documented a nine-month human pregnancy using pictures and accompanying text written by doctors Axel Ingelman-Sundberg, Claes Wirsen and translated by Britt and Claes Wirsen and Annabelle MacMillian. Critics lauded A Child Is Born for its photographs taken in utero of a developing fetus. Furthermore, the work received additional praise for what many described as simple and scientifically accurate explanations of complicated processes during development.

In his 1991 article Screening for Congenital Hypothyroidism, Delbert A. Fisher in the US reported on the implementation and impact of mass neonatal screening programs for congenital hypothyroidism (CH) from the early 1970s through 1991. CH is a condition that causes stunted mental and physical development in newborns unless treatment begins within the first three months of the newborn's life. In the early 1970s, regions in Canada and the US had implemented screening programs to diagnose and treat CH as quickly as possible after the infant's birth. By 1991 many other countries had adopted the early screening program, and Fisher estimated that 10 to 12 million newborns per year were tested in the early 1990s. The screening programs, along with physician education and improved screening techniques, such as radioimmunoassay, helped significantly reduce the incidence of abnormal newborn development resulting from untreated congenital hypothyroidism.

The Sex-determining Region Y (Sry in mammals but SRY in humans) is a gene found on Y chromosomes that leads to the development of male phenotypes, such as testes. The Sry gene, located on the short branch of the Y chromosome, initiates male embryonic development in the XY sex determination system. The Sry gene follows the central dogma of molecular biology; the DNA encoding the gene is transcribed into messenger RNA, which then produces a single Sry protein. The Sry protein is also called the testis-determining factor (TDF), a protein that initiates male development in humans, placental mammals, and marsupials. The Sry protein is a transcription factor that can bind to regions of testis-specific DNA, bending specific DNA and activating or enhancing its abilities to promote testis formation, marking the first step towards male, rather than female, development in the embryo.