Shortly after the Morula enters into the uterus, approximately on the 4th day after fertilization a fluid filled cystic cavity appears in the morula and transforms the morula into a blastocyst.

Oxygenation of fetal blood occurs in the placenta before it returns in the umbilical vein which joins the left branch of the portal vein. It bypasses the capillaries of the liver by going through the ductus venosus, which is obliterated after birth and becomes the ligamentum venosum. The oxygenated blood enters the inferior vena cava and is transported to the right atrium andthen through the patent foramen ovale to the left atrium and on to the left ventricle. From the left ventricle, the blood flows into the aorta and through the fetal vascular network. Blood returning from the head of the foetus passes through the superior vena cava to the right atrium and straight on to the right ventricle and pulmonary artery. However, it does not enter the pulmonarycirculation, being short-circuited by the ductus arteriosus to the aorta. Aortic blood is carried via the umbilical arteries back to the placenta for reoxygenation. At birth, the three short circuits, the ductus venosus, foramen ovale and ductus arteriosus, close.

The development of the kidney proceeds through a series of successive phases, each marked by the development of a more advanced kidney: the pronephros, mesonephros, and metanephros. The development of the pronephric duct proceeds in a cranial-to-caudal direction. As it elongates caudally, the pronephric duct induces nearby intermediate mesoderm in the thoracolumbar area to become epithelial tubules called mesonephric tubules. Each mesonephric tubule receives a blood supply from a branch of the aorta, ending in a capillary tuft analogous to the glomerulus of the definitive nephron. The mesonephric tubule forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows through the mesonephric tubule and is drained into the continuation of the pronephric duct, now called the mesonephric duct or Wolffian duct. The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to the cloaca.

Trigeminal nerve has three divisions, the first is the ophthalmic division that does not originate from any of the pharyngeal arches, the second and third divisions, namely, the maxillary and the mandibular region develop from the first pharyngeal arch.Pharyngeal Arches:1st = Trigeminal V2 & V3 (CN V)2nd = Facial (CN VII)3rd = Glossopharyngeal (CN IX)4th and 6th = Vagus (CN X)

The origins of the neural tube and the nape of the neck where nuchal translucency measurements are taken are from embryonic ectoderm.The structural development of the head and neck occurs between the third and eighth weeks of gestation. The 5 pairs of branchial arches, corresponding to the primitive vertebrae gill bars, that form on either side of the pharyngeal foregut on day 22 are the embryologic basis of all the differentiated structures of the head and neck. Each arch consists of 3 layers: an outer covering of ectoderm, an inner covering of endoderm, and a middle core of mesenchyme. These arches are separated further into external, ectoderm-lined pharyngeal clefts and internal, endoderm-lined pharyngeal pouchesA population of ectodermal cells adjacent to the neural fold and not included in the overlying surface (somatic) ectoderm gives rise to the formation of the neural crest. These neuroectodermal crest cells are believed to migrate widely throughout the developing embryo in a relatively cell-free enriched extracellular matrix and differentiate into a wide array of cell and tissue types, influenced by the local environment. Most connective and skeletal tissues of the cranium and face ultimately come from the derivatives of neural crest cells.

The arches are numbered according to their proximity to the head i.e. the 1st is the closest to the head end of the embryo and the 6th closest to the tail end as shown by the diagram below the table

Shortly after the morula enters the uterus, around the 4th day after fertilization, a clear cystic cavity starts forming inside the morula. The fluid passes through the zona pellucida from the uterine cavity and hence forms the blastocyst.

The differentiation between type 1 and type 2 epithelia in the developing lung buds is evident from 16-24 weeks of gestation.Development of the respiratory tract has five stages:- Embryonic (at 4 – 5 weeks of gestation):Formation of lung buds, trachea and mainstem bronchi occur. These structures are formed from a ventral outpouching of foregut pharynx. At this stage, the beginnings of the five lung lobes are present.- Pseudoglandular (at 5 – 16 weeks of gestation):Formation of terminal bronchioles, cartilage and smooth muscles occur in this stage. – Canalicular (at 16-24 weeks gestation): Differentiation of type I and II epithelial cells can be done in this stage. There is also an increase in the size of proximal airways).- Saccular (at 24 – 40 weeks of gestation): Terminal saccule formation occurs. Production of surfactant takes place at this stage with an increase in the number of goblet cellsUp to half the adult number of alveoli are in place by this stage.- Alveolar (occurs between 32 weeks of gestation till the post-natal age 8): Formation of alveoli and septation occurs with the expansion of air spaces.

There are 6 pharyngeal arches, the fifth does not contribute any useful structures and often fuses with the sixth arch. The structures arising from the aortic arches are as follows:First aortic arch: It regresses except for a very small part that gives rise to the maxillary artery.Second aortic arch: It regresses except for a very small part giving rise to the stapedial artery.Third aortic arch: This arch is the source of the common carotid artery and the proximal part of the internal carotid artery, and the external carotid which arises as a bud from this arch.Right Fourth aortic arch: Is the genesis of the proximal part of the right subclavian artery.Left Fourth aortic arch: Gives rise to the medial portion of the arch of the aorta.Fifth aortic arch: The fifth aortic arch regresses completely and very early in the development.Sixth aortic arch: Either of the sixth aortic arches divides into ventral and dorsal segments, and therefore, their derivatives also divide into these two segments.Right Sixth Arch:Ventral: Gives rise to the right pulmonary artery.Dorsal: It degenerates completely and loses its connection with the dorsal aorta.Left Sixth ArchVentral: It gives rise to the left pulmonary artery that goes to the left pulmonary bud.Dorsal: It forms a vital connection during intrauterine life between the left pulmonary artery and the arch of the aorta. This structure is called ductus arteriosus.

Functional closure of the ductus arteriosus is neonates is completed within the first few days after birth. It normally occurs by the 12th postnatal week. It has been suggested that persistent patency of DA results from a failure of the TGF-B induction after birth. Due to increased arterial pO2, constriction of the DA occurs. In addition to this on inflation the bradykinin system is activated with cause the smooth muscles in the DA to constrict. A decrease in the E2 prostaglandin is also an important factor as raised levels have been indicated in keeping the patency of the DA.