In class this week, we discussed the idea of phylotypic stage. In many previous courses, including biology and psychology courses, I was always taught that early embryos of separate species look extremely similar. While this is partly true, the artistic representation (Image 1) that has always been displayed in class is exaggerated. The 'partly true' portion of this phylotypic stage, however, is quite interesting to me. You can see a realistic version of the phylotypic stage in Image 2.
During the phylotypic stage, structures are similar across very different species including the notochord, somites, and neural tube. So, how do such similar structures develop into such unique organisms? Well, it is theorized that as the phylotypic stage progresses, Hox genes are expressed which specify positional identity. The pattern of Hox gene expression is very different across species, which allows for specialized body plans and unique organisms. Therefore, the phylotypic stage tapers off as species become less similar, much like the hourglass model seen in Image 3.