Through Rathke's work the real embryonic archetype of the vertebrate skull was for the first time disclosed. Rathke discussed this archetype and its relation to the vertebral theory of the skull in another paper of the same year (1839), but before going on to this paper, we shall quote from the paper on the adder the following passage, remarkable for the clear way in which the idea of the embryological archetype is expressed. "Whatever differences may appear in the development of Vertebrates, there yet exists for the different classes and orders a universally valid idea (plan, schema, or type) ruling the first formation of their separate parts. This idea must first be worked out, though possibly with modifications, before more special ideas can find play. The result of the latter process, however, is that what was formed by the first idea is not so much hidden as partially or wholly destroyed" (p. 135). Rathke's general paper on the development of the skull in Vertebrates[211] treats the matter on a broader comparative basis than his paper on the adder, and takes into account all the vertebrate classes, in so far as their development was then known. He here makes the interesting suggestion, later entirely confirmed, that the _basis cranii_ or basilar plate is first laid down as two strips, one on each side of the chorda--the structures now known as parachordals (pp. 6, 27). For this supposition, he thinks, speaks the structure of the skull in _Ammocoetes_, which in this respect is the simplest of all Vertebrates (pp. 6, 22). In _Ammocoetes_, as Johannes Mueller had shown, the foundation of the skull is formed by two long cartilaginous bars, between the hinder portions of which the notochord ends. In these Rathke was inclined to see the homologues of his trabeculae, and of the parachordals which he was ready to assume from his embryological observations.