Before there vertebrae, there was a notochord. Notochords evolved in invertebrates known as chordates and there are a few living examples of primitive chordates, such as lancets and tunicates. The most primitive fish today, the jawless fish, do not have full-fledged vertebrae, and still depend on the notochord as their primary axial support.
Even once vertebrates began to develop vertebrae, these structures arose in pieces around a notochord which still ran uninterrupted from head to tail.
Human embryos possess a notochord running from head to tail as their initial longitudinal support.
During the fetal development of higher vertebrates (including humans), the notochord develops cranially until it reaches the site of the mouth. It establishes the long axis of the embryo, provides support, and sends signals to surrounding tissues. The nerve chord develops dorsal to it and vertebrae develop around it. In adults, remnants of the notochord form the nucleus pulposus of the intervertebral disks and additional remnants in some individuals may cause benign or malignant chordomas. (Moore, p. 69-70).
Below are pictures of the notochord in developing amniote embryos.
Despite the differences between the brains of adult placental mammals (such as humans) and the brains of more primitive vertebrates, the brain of placental embryos show a number of similarities with the more primitive condition. The brain develops as a series of linear regions which slowly fold into the adult configuration. The midbrain is prominent and exposed and its channel for cerebrospinal fluid is much more prominent than in adults..
Cat fetal Brains
In reptiles and marsupials, there is no corpus callosum uniting the cerebral hemispheres. This condition also exists in the fetal brain.
Two small bones associated with the embryonic jaws are converted to middle ear bones and enclosed in a portion of the temporal bone, just as is documented in fossil mammal-like reptiles. The auditory ossicles form in the first half of fetal development but they are surrounded by mesenchyme until the eighth month at which point the mesenchyme dissolves (Sadler, p.334).
The cochlea begins as a straight structure (as in more primitive vertebrates and gradually becomes coiled).
VOMERONASAL SYSTEM (VNS)
Higher vertebrates typically have a second olfactory system called the vomeronasal system. Human fetuses develop this system but much to most of it degenerates in most humans. Although it is not yet certain whether this can function in some humans, it is generally concluded that in most humans this system is nonfunctional.
VOMERONASAL ORGAN (VNO)
The vomeronasal organ is a pair of structures on either side of the nasal septum (vomer) near the base. Vomeronasal ducts are openings which range from a tenth of an inch to invisible to the naked eye to being absent. A study on the percentage of the human population with pits opening into these ducts concluded that 16% of people retain them; a second study that used an endoscope concluded this number was 76%. The lining of duct is pseudostratified columnar epithelia with kinocilia and microvilli with myelinated and unmyelinated neurons under the basement membrane. This is anatomical arrangement is unique in the body. In some animals, these ducts connect to oral cavity as well (carnivores, ungulates). A capsule made of cartilage encloses these ducts.
Most people seem to have a vomeronasal organ. It consists of neurons which constantly regenerate, just as in the main olfactory system. Neuron specific markers do bind to cells in the VNO (i.e. the VNO has neurons) and they make projections into the brain. These neurons have microvilli, not the cilia observed in the main olfactory system. Some putative pheromones blown onto the VNO (not MOS) caused measurable electrical signals from VNO. This organ has a lumen.
Each ligand examined stimulates a nonoverlapping set of neurons in the VNO. The thresholds of some VNO neurons for pheromones are 10-11 M, and are thus among the most sensitive mammalian chemoreceptors. Two distinct multigene families G protein coupled receptors compose the vomeronasal receptors. These gene families are V1r and V2r; rats may have 100 genes in these two families; another paper states more than 240 family members. There are 8 homologs in the human genome but some may be nonfunctional pseudogenes; one human homolog V1RL1 is an active human gene in a diversity of ethnic groups but it is expressed only in the main olfactory epithelium.
Human embryos at stage 17 have no evidence of a VNO; all embryos at stage 18 or later have. It continues to develop and its full size not achieved by birth although an earlier study concluded that VNO shrinks during fetal development and may not have neurons by birth. Some vertebrates lack a VNO: it is absent in birds and adult catarrhine mokeys. Only 1 of 18 bat families have; in some bats it is even absent in embryo. The flehmen behavior ( a kind of grimace displayed after some mammals sample urine conspecific urine) may make VNO ducts more accessible to pheromones.
Many animals are segmented; their anatomical structure has been achieved through repeating a basic unit of structure which then can be modified. Although segmentation is not overly obvious in many adult vertebrates, it is much more apparent in their embryos.
By the 3rd week of human embryonic development about 38 pairs cuboidal blocks of mesodermal tissue known as somites form. By the 5th week there are 42-44 pairs of somites. Most of the axial skeleton and skeletal muscles will be derived from these somites. Many somites in the tail will later degenerate (Moore, p. 74).
The heart is believed to have evolved from a pair of contractile blood vessels (as observed in many simple animals) which fused. In human development, the heart begins its development as a pair of endocardial tubes which fuse.
This straight tube divides into four regions: a sinus venosus, a single atrium, a single ventricle, and a truncus arteriosus. These 4 regions persist in the adult heart of lower animals; adult human hearts do not retain a truncus areteriosus or a sinus venosus.
When the pericardium forms, the atrium and sinus venosus are located outside the pericardial cavity (Sadler, p.182). The sinus venosus remains paired longer than the other regions of the heart. Initially there are valves around the sinoatrial junction (Sadler, p. 185-6).
In human development, the single atrium divides into a left and right atrium; the single ventricle divides into a left and right ventricle. The same process occurred through evolution when the single fish atrium divided into left and right atria in amphibians and the single reptilian ventricle divided into ventricles in mammals.
There are a number of problems which can result from the unequal division of the ventricles (interventricular septum defects) or of the truncus arteriosus. The division of the truncus can be unequal in the tetralogy of Fallot. Eight in 100,000 births possess a persistent truncus arteriosus (Sadler, p. 200-4).
. The embryonic human circulatory system includes prominent aortic arches (which exist in adult fish) which service the pharyngeal arches.
By the end of the 5th week there are three major pairs of veins: the vitelline, umbilical, and cardinal veins. The anterior and posterior cardinal veins drain into the common cardinal vein (Sadler, p. 215-6). In the fifth through seventh weeks, the subcardinal veins drain the kidneys and the sacrocardinal veins drain the hind limbs. The supracardinal veins develop to drain the body wall and substitute for the posterior cardinal veins (Sadler, p. 218-9). These veins must be reorganized to form the venous system of adult humans.
In some people, there is a double inferior vena cava starting in the lumbar region because the left sacrocardinal veins is still connected to the left subcardinal vein. Some people lack an inferior vena cava because the right subcardinal vein (what should be the renal segment of the inferior vena cava) does not fuse to the hepatic segment but instead enters the superior vena cava by way of the supracardinal 9azygos) vein. Other abnormalities include a double superior vena cava and a vena cava which is derived from the left cardinal system rather than the right (Sadler, p. 219-220).