There seems an obvious divide to me from the first month when all the groundwork is being laid to the second month when our adorable, tiny organism starts doing the serious work of cranking out brand new organs. If you look at any information that goes week by week in embryonic development it varies, so I’ve tried to stick with one or two sources that don’t contradict each other. I’ll point out major highlights for a given week and also spend some time going into a specific organ or system in more detail in a week that is relevant to it. But in many cases when an organ actually starts developing is up for debate since the three embryonic layers give rise to everything and there is so much work to be done over the course of the fetus's stay in the womb. Some organs have an initial structure develop early on but the organ itself isn’t constructed until weeks later for example.
When discussing early brain and spinal cord development two structures are key- the neural tube and the notochord which are very early beginnings of the two, respectively. The neural tube, comes from the ectoderm which forms the neural groove, the groove deepens and elevates as it grows and is then referred to as the neural folds, eventually these folds meet up to form a tube with a cavity in the center, this overall process is known as neurulation and gives rise to the neural tube. The neural tube positions itself along the dorsal-ventral (back-belly) axis to dictate the specific types of neurons the nervous system development will give rise to. Ultimately the neural tube will give rise to the brain and the spinal cord. The notochord is a transient, embryonic structure made from the mesoderm in several early permutations until it is referred to as the notochord. It is a critical part of the embryo as it influences the patterns and signals of all surrounding tissue. It releases the protein named sonic hedgehog (arguably the most random/coolest named protein ever) which influences signal transduction of tissue differentiation and development. (The notochord gives rise to a permanent structure in the spine- specifically a jelly-like substance found in the intervertebral discs).
Somites are mesodermal masses that form on either side of the neural tube and give rise to the vertebra and associated structures (muscle, dermis, cartilage) as well as some additional anatomy like the dorsal aorta. Also in this picture above, it’s important to note the neural crest in green that becomes the neural crest cells and ultimately the spinal ganglion, also known by its more common name the dorsal root ganglion. The neural crest cells are transient, like some of the other structures mentioned in this post, and they give rise to a variety of cells some obvious (various neurons and supporting glia) and others not so obvious (melanocytes, some cartilage, muscle and bone). For now that is all I’ll say about these things until we discuss these systems in a bit more detail.
Before we can move onto the individual organ systems, more groundwork needs to be laid in the developing embryo. A major development is happening in the yolk sac around the fourth week thanks to folding and rearranging in the embryo. Ultimately the yolk sac and its innermost layer the endoderm will fold up and give rise to the gut tube. This tube will be the starting off point for the organs of the digestive tract, as well as the lungs. The gut tube will quickly be known by its three separate regions the foregut, midgut, and hindgut where each section matches up with organs in descending order. For example the foregut gives rise to the trachea and esophagus, the midgut gives rise to most of the stomach and intestines and the hindgut gives rise to the end of the tract- most of the colon, the anus, and rectum. This is an insanely oversimplified tale but I don’t want to get bogged down with all the layers and folding and nomenclature. The gut tube represents one of the major tubes formed in early development along with the neural tube and the notochord, and a little bit later the heart tube, and the lung tube. Your growing baby is a bit like a balloon animal artist- they always seem to start out with the same basic structure and go from there. The little organism starts by making a tube and then specifies it from there to make the heart, the lungs, the stomach, etc. What distinguishes the gut tube from other tubes made in early development is how it gives rise to the majority of the organs found in the body. We’ll look at organ development on an individual basis but this provides the starting off point. I will say again a lot of this distinction and differentiation is made possible by patterning based on varied gene expression, like our aforementioned friend the sonic the hedgehodge gene (there are other critical ones as well but seeing as they aren’t named after Mario and Luigi I don’t feel the need to mention them). Also a lot of reorganizing goes on to get the embryo from a three layered flat structure to an intermediate mushroom looking structure (with the amniotic cavity resembling the cap of the shroom and the yolk sac resembling the stem) to a little miniaturize baby where there is a clear head location and cute little baby bum location. All of this happens starting at the 4 week and by the 11th week the baby actually looks like a baby.
The above picture illustrates my mushroom metaphor and it also shows the early formation of the gut tube (from a cross section view) and puts the gut tube into perspective with the above mentioned neural tube (shown here in blue) with the flanking somites, as well as the notochord the black dot below it.