162 THE KMBRYOLOGICAL CRITERION 



bones in front of the basisphenoid. The cranium might 

 indeed be divided upon ossification into a series of segments 

 bearing a more or less remote analogy with vertebra?. " In 

 the process of ossification there is a certain analogy between 

 the spinal column and the cranium, but that analogy becomes 

 weaker and weaker as we proceed towards the anterior end 

 of the skull" (p. 585). The best way to state the facts is to 

 say that both skull and vertebral column start in their 

 development from the same point, but immediately begin to 

 diverge. The clear indications of segmentation which fully 

 ossified adult skulls undoubtedly show are, therefore, 

 secondary, and the vertebral theory of the skull, which was 

 originally based upon the appearance of such fully ossified 

 crania, is on the whole negatived by embryology. 



We have now to turn back a few years in order to 

 follow up another line of discovery which had an important 

 bearing upon the theory of the vertebrate skull the working 

 out of the distinction between membrane and cartilage 

 bones. 



As early as 1731, R. Nesbitt, 1 in two lectures delivered 

 to the Royal College of Surgeons, demonstrated that in the 

 human fiutus some bones were formed not in cartilage but 

 directly in fibrous tissue, and this observation was confirmed 

 by other human anatomists, particularly by Sharpey at a 

 considerably later date. In 1822 Arendt- focussed attention 

 upon the remarkable structure of the skull of the Pike, with 

 its cartilaginous brain-box studded all over with bony 

 plaques, an arrangement which had already attracted the 

 interest of Cuvier and Meckel. K. E. von Baer :! in 1826 

 discussed at some length the relation between the bony and 

 the cartilaginous skull in fishes, with particular reference to 

 the sturgeon, coming to the following just conclusion : " If 

 we consider the fibrous skeleton of Ammocoetes as the first 

 foundation of the skeleton of Vertebrates, we can form a 



1 Human Ostcogcny e.\-/>l<iin<-tt in two Lectures, London, 1736. 



l>, ,v////.v ossci Rsocis lucii structura sinul<iri. nisscrt. inaug. 

 Rejjiomonti, 1X22. 



:) "Uebcr das ausscrc und inncrc Skclct," Meckcl's /Itr/iiv, pp. 327- 



