It is my understanding that in gastrulation epiblast cells migrate to the primitive streak where they start to form the three germ layers; does this mean that gastrulation only occurs at the caudal half of the developing embryo
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$\begingroup$ 1. What organism are you asking about? 2. Where are you getting the idea that the primitive streak is only in the caudal half? 3. What do you understand gastrulation to mean? $\endgroup$– De NovoDec 20, 2019 at 19:46
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$\begingroup$ Humans. It came up in one of my lecture notes and in my understanding gastrulation is the formation of the three germ layers (ectoderm, mesoderm, endoderm) from the epiblast $\endgroup$– Fazal SyedDec 21, 2019 at 22:15
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It would help a little in answering your question to know more about what organisms you are interested in, or the specific source that you are referencing, with e.g. the relevant quote.
For example, the primitive streak is specific to amniotes: avians, mammals, and reptiles. In other common model organisms such as flies or amphibians gastrulation happens differently.
Another thing to keep in mind is that gastrulation is a very large and structurally distending process, so it can't be really said to happen "only on one end" of the embryo. The whole embryo is going through gastrulation. We can however talk about the placement of certain gross morphological structures on the embryo. But really each cell is affected by gastrulation, whether it's immediately obvious under a microscope or not.
Looking at the Amniote section of the wiki page on gastrulation, it appears that the main morphological changes in gastrulation do occur on the caudal side - for amniotes.
However, in Drosophila, gastrulation involves the formation of an approximately homologous structure called the ventral furrow, which is obviously on the ventral side, extending most of the anterior-posterior axis. However the invagination event does actually happen on the dorsal/caudal side.
So, it's complicated, and it depends a lot on which organism you're talking about.
Update following response
Humans are amniotes, they follow more or less the normal amniote path in gastrulation according to the wiki link above.
An important note missing in my previous answer above is that gastrulation itself defines bilateral symmetry in embryos; prior to this establishment animal-vegetal is the only obvious axis. So we are kind of defining the rostral-caudal axis according to the formation of such structures as the primitive streak, which are formed during gastrulation.
So it's a little circular to say that gastrulation happens on caudal end only; as gastrulation includes the step that establishes the rostral-caudal axis. Without gastrulation that axis just doesn't exist.
Again, gastrulation is a process that happens to the entire embryo. It doesn't just happen in one area. Structures such as the primitive streak are just obvious clues that the process is happening. It might be proper to say that the primitive streak begins forming caudally (I don't know of exceptions, though I'm not an expert), but it would not be proper to say that gastrulation is a caudal process.
answered Dec 20, 2019 at 19:52
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$\begingroup$ Hi thanks for the answer I meant in humans $\endgroup$ Dec 21, 2019 at 22:14
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1$\begingroup$ Your point that gastrulation defines the axes is a good one. I would also note that the organizing node and primitive streak are not "in the caudal half" as the OP says. They move, which is a key point. Specifically, they move in a rostrocaudal direction (or craniocaudal if you prefer), the axis is established based on that movement. Early cells that pass through the streak form more rostral structures. Later cells form more caudal structures. $\endgroup$– De NovoDec 26, 2019 at 4:17
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$\begingroup$ I think the Wikipedia article has confused the OP with poor word choice when it says the primitive streak forms "toward the caudal or posterior end." It doesn't form at the caudal end. It moves towards that end as gastrulation proceeds $\endgroup$– De NovoDec 26, 2019 at 4:20