06 



EMBRYOLOGY. 



vertebral column, especially in the manner in which the metamor 

 phosis into cartilaginous tissue takes place in the membranous 

 primordial cranium. In both the choiidrification first begins at the 

 surface of the chorda dorsalis (fig. 329 A). 



As a foundation for the base of the skull there arise two pairs of 

 elongated cartilages : behind, on either side of the chorda, the two 

 parachordal cartilages (PE) ; in front, the two trabeculce cranii (TV) 

 of RATHKE, which begin at the tip of the chorda and from there 

 run forward beneath the between- and the fore -brain. 



Tig. 329 A and B. First fundament of the cartilaginous primordial cranium, from WIEDEKS- 



HEIM. 



A, First stagt. C, Chorda ; PE, parachordal cartilage ; Tr, RATHKE'S trabeculae cranii ; PR, 

 passage for the hypophysis ; N, A, 0, nasal pit, optic vesicle, otocyst. 



B, Second stage. C, Chorda ; B, basilar plate ; T, trabeculse cranii, which have become united 



in front to constitute the nasal septum (S) and the ethmoid plate ; Ct, AF, processes of the 

 e:hmoid plate enclosing the nasal organ ; 01, foramina olfactoria for the passage of the 

 olfactory nerves; PF, post-orbital process; NK nasal pit A,\ 0, optic and labyrinthine 

 vesicles. 



The four pieces soon fuse with one another (fig. 329 B). The two 

 parachordal elements grow around the chorda, first below, then above, 

 thus enveloping it and producing the basilar plate (B}. Its anterior 

 margin rises far up into the angle of the flexure betwe?n mid-brain 

 and between-brain and corresponds to the future dorsum sells3. The 

 trabeculce cranii (T) spread out at their anterior ends, which become 

 fused to constitute the ethmoid plate (&), the foundation of the 

 anterior portion of the cranium, which acquires its particular 

 stamp through its reception of the organ of smell. In the middle of 

 their length they remain separate a long time, and enclose an opening, 



