DEVELOPMENT OF FIFTH DAY 



539 



On each side of the notochord a sheet of cartilage develops. These 

 wo sheets are known as parachordal plates (Fig. 310). They form a 



tr 



%>-'' 



,o.ch. 



p.ch 



p.C.h. 



tectcn 



bas. cr 



- ---sph.lat. 



tect s.at 



Fig. 310. 



Diagrams of skull formation in Salmon. A, First anlage of cranium. B, C, D, 

 successive stages in cranial development. Left half of D is an advance in 

 development of right half, au, eye; bas.cr., base of cranium; e.b.c.a., e.b.e.p., 

 anterior and posterior basicapsular commissure (ascending process of palato- 

 quadrate cartilage) ; ch, notochord; c.tr, trabecular cornu ; hyp, opening in which 

 hypophysis develops ; n.k., nasal capsule ; o.bl., and f.b.c., fenestra basicapsularis ; 

 o.k., ear capsule; occ, occipital region; p.ch., parachordal plates; sph.lat., lateral 

 sphenoid; tect.cr., roof of cranium; tect.s.ot., Cartilaginous arch between otic 

 capsules representing the cartilaginous roof of higher vertebrates (tectum 

 synoticum) ; tr., cranial trabeculae. (A, D, after Waskoboynikow ; B, C, from 

 Gaupp after Stohr's model.) 



