TBH think about the orientations of the germ layers. The ectoderm is the outer "visible" region which forms stuff we can see, like the eyes, skin, eyes, anus, tooth, nervous system, etc. The endoderm, as the name implies, is the inner segment of the 3 germ layers and thus forms the INNER linings of many systems plus some random organs like the liver and pancreas (these two go hand in hand so its easy to remember that these two are part of hte endoderm. plus the products of these two are excreted into the lumen of the GI tract which is formed by the INNER LININGS). Mesoderm forms almost every connective tissue like muscles, bones, dermis, circulatory system plus gonads (think about gonads as connecting your genes to the next generation). TBH it was hard for me too at first, but after writing the list down many times, I managed to remember it (even now, after 8+ months after finishing the DAT).
