Rudolph Leackart (1822-1898) was one of the most prominent zoologists of the 19th Century, and is considered to be the father of parasitology. In 1869, Leackart was awarded a full professorship at the University of Leipzig. He developed these beautiful wall charts as teaching aids.
The top left wall chart shows the early stages of the development of the nervous system in a fish embryo. The bottom middle drawing in that chart shows primary neurulation, during which the neural plate starts to invaginate. This produces a crease on the surface of the embryo visible in Leuckart’s drawing as the dark furrow down the centre of the embryo. As the crease gets deeper, the edges, or neural folds, come together and then fuse to form the neural tube. During secondary neurulation, the neural tube is hollowed out. One end of the neural tube will eventually form the brain; the other will form teh spinal cord.
(Via BibliOdyssey; see the rest of Leuckart’s wall charts at the Marine Biological Laboratory Woods Hole Oceanographic Institution library.)