126 STRUCTURE OF THE VERTEBRATES 



chondrocraniiim is phylogenetically and ontogenetically older^ 

 and will be discussed first. 



The embryonic chondrocraniiim of the higher vertebrates has 

 its counterpart in that of the cyclostome as an open trough in 

 which the brain lies, with a cartilage bridge over the nerve cord 

 at the posterior margin. The cupped hands with the tips of the 

 thumbs touching will assist in visualizing the structure. In addi- 

 tion to the original trough there are three sensory capsules which 

 develop synchronously. The most anterior capsule surrounds the 

 olfactory nerves; the middle one is the eye, or optic, capsule; and 

 the most posterior is the ear, or otic capsule which encloses the 

 organs of equilibrium, and the true ear of the higher vertebrates. 

 Olfactory and otic capsules form an integral part of the chon- 

 drocranium, but the eye capsule always remains free, leaving the 

 eye movable. 



The development of the chondrocranium is from two pairs of 

 ventral cartilages, and three pairs of capsules, each arising as a 

 separate cartilage center. The ventral cartilages lie on either side 

 of the notochord, the posterior pair being the parachordal (be- 

 side the notochord), and the anterior are the prechordal. The 

 three pairs of capsules arise slightly later. As the ventral carti- 

 lages grow, they fuse ventrally, surrounding the notochord, but 

 leaving a ventral hole immediately anterior to the notochord. 

 This opening remains in the higher forms as the pit for the pitui- 

 tary gland. 



The lateral margins of the basal cartilages then grow dorsally. 

 In the meantime the olfactory capsule has surrounded the olfac- 

 tory center, and the otic has surrounded the ectodermal ear 

 structures. These soon fuse with the chordal cartilages to form a 

 coherent unit. Cartilages then grow from either otic capsule to- 

 ward the dorsal-median line and join to make the posterior 

 ^'bridge" or sijnotic tectum, and the chondrocranium is complete. 

 The posterior opening through which the nerve cord passes is the 

 foramen magnum, and numerous smaller foramina are caused 

 by the passage of nerves and blood vessels from the brain 

 through the chondrocranium to the body tissues. 



The dogfish chondrocranium is in many ways specialized. In 

 addition to the simple trough a roof has developed, completely 

 enclosing the brain except for the foramina. Also, a rostrum has 



