Contribution to the Embryology and Phylogeny of the Pycnogonids, Vol. 5 (Classic Reprint): A Dissertation, Presented to the Board of University Studies for the Degree of Doctor of Philosophy

Excerpt from Contribution to the Embryology and Phylogeny of the Pycnogonids

It is here seen that the ectoblast cells over one hemisphere are somewhat higher than at the opposite, and I find this very con stant in sections of both species at this stage of development. After this it becomes difficult to follow out the fate of the germ layers. The outer cells become smaller and ?atter to form the ectoblast and the-inner cells arranged into organs, the most con spicuous of which is the digestive tract. There is a triangular invagination to form the stomodaeum, and the proboscis appears between the first pair of appendages, which has now begun to form. These appendages are very conspicuous in surface views, where they project beyond the surface of the body. Between them appears the slightly projecting proboscis, and about the middle of the embryo are. Seen the second and third pairs of appendages, which are small and inconspicuous. Dohrn has given excellent figures of embryos at this stage, both in his earlier paper and in his later monograph. Soon after this the egg coverings swell up somewhat and the embryos fin ally break out of the egg case, so that the appendages now can straighten out.