The allantoic artery and vein are called the umbilical artery and vein in placental mammals. Notice that the umbilical artery and vein form from the mesodermal part of the allantois layer of the placenta. Mammals have the same three circulation routes that we saw in birds. The vitelline route goes to/from the yolk sac and the embryo, the allantoic route goes to/from the placenta, and the intraembryonic route is within the embryo.
Pulling all the pieces together
When the embryo develops its digestive tract, ectoderm at the ends of the tract fuses to endoderm lining the tract. The ends of the tracts, which are derived from ectoderm, are the stomodeum, or what will become the mouth, and the proctodeum, or what will become the anus. Initially these are outside the little embryo's digestive tract. The tract itself starts out as the foregut (pharynx to stomach), midgut (small intestine), and hindgut (large intestine and rectum). The first part of the foregut is the pharynx. The oral plate is a thin band of tissue that separates the stomodeum and foregut. It will later rupture to form the margins of the mouth. A similar thing happens with the anal plate.
When you look at where the sensory structures of the nose, eyes, and ears are located you see that they start out within the neural tube. Although we think of these structures as occurring outside the brain, the fact that they start as part of the neural tube means that their neurons are similar to those in the brain and therefore cannot regenerate later in life.
The otic vesicle is the inner ear. It develops from an auditory placode that sinks from the surface into the head. The sensory part of the nose starts as a nasal placode.
When you look at an early embryo much of the tissue in the head is mesenchyme, undifferentiated mesoderm. This will eventually become connective tissue and muscle.
The pharyngeal area of the chick or mammal is much different in an embryo than in an adult. This area starts as pharyngeal (visceral) arches and clefts, which are equivalent to gill arches and clefts. The lateral part of the pharynx forms pharyngeal pouches. The anterior ones form first. At 48 hr only the first visceral arch and pouch have formed. By 72 hr more have formed.
The first visceral arch becomes the mandibular arch and maxillary processes. The maxillary processes at this point are anterior extensions of the mandibular arch. These structures are destined to become the upper and lower jaws.
The visceral arches have aortic arches running through them. The visceral arches are solid tissue and the aortic arches are blood vessels. The aortic arches take blood from the ventral aorta to the dorsal aorta. Like the visceral arches, the aortic arches start developing from the anterior end. There are a total of 6 of them, but in amniotes the anterior ones are normally gone before the posterior ones develop.
In the early embryos we've been looking at there are 2 dorsal aortae - one on each side. By 72 hr the anterior part of these vessels has fused together to make a single dorsal aorta, but it is still paired at the posterior end of the embryo.
The anterior cardinal vein and the posterior cardinal vein drain the anterior and posterior parts of the embryo. The vessel that forms by the joining of these two is the common cardinal vein. It takes blood to the heart.
At about 33 hr we see a new type of mesoderm developing in the chick. Intermediate mesoderm is going to give rise to the urogenital system. It sits between the somites and laterall mesoderm. You can see a mesonephric duct by 72 hr. It is the tube that drains the kidney.
The somite starts as a solid piece and at 33 hr looks like a triangle. By 48 hr one can see subdivisions of the somite. At this point it can be divided into a dermo-myotome and sclerotome. Later the dermo-myotome divides to become the dermotome and the myotome. The sclerotome becomes hard tissue, the dermotome becomes dermis, and the myotome becomes muscle.
Cells called hemangioblasts develop from splanchnic mesoderm in the area opaca. These form clusters called blood islands. The cells that sit on the outer part of the blood islands become the precursors of blood vessel cells and cells within the islands become the stem cells that give rise to all other types of blood cells. The yolk sac also produces some of the primitive blood cells.
Hans Spemann's Experiments
In the 1920s Hans Spemann and some of his graduate students
were investigating the mechanism by which embryos become
differentiated. We know that at the 2 cell stage that if each
blastomere is separated, each can grow into a normal individual.
In some cases even later stages can grow into complete individuals.
The ability to do this at later stages is limited by the amount of
cytoplasm in the cell.
Spemann took a newt egg that was fertilized and before it
divided into 2 blastomeres he took a strand of baby hair and made
a loop and put over the egg. He tightened the loop and left it
such that one side had all the nucleus. When cleavage occurred all
of the divisions occurred in the half with the nucleus. If a cell
managed to get through the loop, division would then occur on the
other side of the loop too and in some cases 2 embryos would
develop. This showed that the nucleus of even these later cells
had all of the information needed to create an embryo.
If we start with undifferentiated cells in the blastula and
proceed to primary tissue layers in the gastrula stage what causes
these tissues to become certain structures in exactly the right
place? Hilde Mangold, Spemann's student, in the 1920s took a
dorsal lip of the blastopore and transplanted it onto a host
embryo. Cell movement occurred around both the host and introduced
dorsal lip and the result was a host embryo with a partially
developed secondary embryo. The development of this secondary
embryo was due to the graft. Cells moved into the blastopore and
developed into the same structures as we saw in a normal embryo.
Think back to what you know about amphibian embryos. The
cells that move into the blastopore and form the roof of the
archenteron are the cells that become the mesoderm and notochord.
It is their presence under the ectoderm cells that causes the
ectoderm to differentiate into neural plate cells. This process is
called induction. The structure that causes this to happen is the
Induction of the neural system is the result of the ectoderm
sitting on top of the presumptive notochord and somites. The
anterior part of the nervous system (brain) is induced by the roof
of the archenteron infront of these structures. Only grafts from
the dorsal lip of the blastopore and surrounding cells could cause
induction to start.
The ability of cells to react to an inductor is greatest in
early gastrulation, still high in mid-gastrulation, and declines
late in gastrulation. By neurulation it is fading away.
The dorsal lip was dubbed the primary organizer by Spemann in
1938. This organizer originates from cells that were derived from
the gray crescent area and is only found on the dorsal part of the
embryo. The dorsal lip sits at a point where the yolkless cells
meet the yolky cells. Those more active, pigmented, yolkless cells
from the animal pole do the moving and the heavier yolky cells
With all of the movement that occurs during the blastula and
gastrula stage, it is hard to follow individual cells. Therefore,
in the 1920s Vogt came up with the idea of taking agar soaked in
vital dye, which doesn't injure the embryo, and putting tiny pieces
on the embryo. The agar is removed and the dye stains the surface
cells of the embryo. These color spots can be observed and
followed. Today we use more sophisticated marking methods.
Last updated on 22 June 2011
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