Mueller was, of course, very ready to accept Rathke's opinions on this subject, for he considered that they supported his own theory of the vertebral nature of the skull. After describing in his _Handbuch der Physiologie_ the cartilaginous bands in _Ammocoetes_ and their highly differentiated homologues in the Myxinoids, he writes in the later editions, "Hence we see that in the cranium, as in the spinal column, there are at first developed at the sides of the chorda dorsalis two symmetrical elements, which subsequently coalesce, and may wholly enclose the chorda. Rathke has recently observed, in the embryos of serpents and other animals, before the formation of the proper cranial vertebrae, two symmetrical bands of cartilage, similar to those which I discovered as a persistent structure in _Ammocoetes_.... At a later period the _basis cranii_ of vertebrate animals contains three parts analogous to the bodies of vertebrae, the most anterior of which, in the majority of animals, is generally small, and its development frequently abortive, whilst in man and mammiferous animals the three are very distinct. These parts are developed by the formation of three distinct points of ossification, one behind the other, in the basilar cartilage."[212] Rathke was very cautious about accepting the vertebral theory of the skull; he saw that the facts of development were not altogether favourable to the theory, and he gave his adherence with many reservations and saving clauses. His general attitude may be summed up as follows.[213] The chorda sheath is the common matrix of the vertebrae and of a large part of the skull. The basilar plate and the trabeculae, which are developed from the chorda sheath, give origin to three bones, which might possibly be considered equivalent to vertebral centra--the basioccipital, the basisphenoid, and the _Riechbein_ (ethmoid). The _Riechbein_ develops from the fused ends of the trabeculae. The presphenoid might also be considered as a vertebral body, but it develops independently of the basilar plate and trabeculae.