DEVELOPMENT OF SKULL: MULLER 143 



in all vertebrate embryos with the persistent notochord 

 which forms the chief part or the whole of the vertebral 

 column in the Cyclostomes. The notochord possesses an 

 inner and an outer sheath and the outer sheath is continuous 

 with the basis cranii (p. 92). It is in the outer sheath that 

 the vertebra; develop from four separate pieces, in fish at 

 least, plus an additional element which helps to form the 

 centrum. The skull of Vertebrates consists, according to 

 Miiller, of three vertebrae, whose centra are the basioccipital, 

 the basisphenoid and the presphenoid. Other bones 

 besides those belonging to the vertebrae are present, but this 

 formation out of three vertebrae gives the essential schema 

 for the skull. Now the brain capsule, like the sheath of the 

 spinal cord, is a development from the outer sheath of the 

 notochord. If the skull consists of vertebrae we should 

 expect the centra of the skull-vertebrae to develop in the 

 outer sheath at the sides of the cranial section of the 

 notochord as two separate halves, just as do the bodies of 

 the vertebrae ; we should expect further the cartilaginous 

 side-walls of the cranium to develop in the membranous 

 brain-sheath just as the neural arches develop in the 

 membranous sheath of the spinal column. In Rathke's 

 discovery (!) of a segmentation of the basis cranii into three 

 parts, and of the isolated formation of the vomer, Miiller sees 

 a confirmation of his view that the skull is composed of three 

 and not four vertebrae. But there is nothing in Rathke's 

 observations to support the idea that the centra of the cranial 

 vertebrae are formed from separate halves. Miiller has to be 

 content with a reference to the state of things in Aunnocoetes 

 (which, by the way, he did not know to be the young of 

 Petromyzon). In the simple skull of Aunnocoetes the base 

 is formed chiefly by two cartilaginous bars lying more or less 

 parallel with the longitudinal axis of the skull and embracing 

 with their hinder ends the cranial portion of the notochord. 

 These bars, declares Miiller, are clearly the still separate 

 halves of the pars basilaris cranii, and represent the divided 

 centra of the two hinder cranial vertebrae. To complete the 

 parallel between the development of the skull and of the 

 vertebrae, it would have been necessary to show that the side 

 walls of the cranium developed in a similar manner from 



