CHONDROCRANIUM OF SYNGNATHUS FUSCUS 435 



Farther posterior the notochord increases greatly in diameter 

 and invades the cavum cranii. The parachordals abut against 

 the ventrolateral surfaces of the notochord, so that it really lies 

 in a groove on the dorsomesial surfaces of the parachordals. 

 The parachordals are separated from each other ventral to the 

 notochord and there is no trace of a hypochordal bridge. 



In the region between the posterior parts of the otic capsules, 

 the enlarged notochord lies between the maculi utriculi and the 

 auditory ganglia adjacent to them ventrolaterally. The relation 

 of the parachordals to the notochord remains as in the more 

 anterior region. x\t the posterior end of the otic capsules the 

 cartilage forming their walls fuses with the occipital processes 

 which lie on the dorsolateral surfaces of the notochord dorsal 

 to the parachordals (fig. 8). The parachordals retain their 

 identity for a short distance posterior to this fusion; gradually, 

 however, the cells of the parachordals become confluent with 

 those of the occipital cartilages (fig. 8). A slender bridge of 

 fibrous connective tissue connects the occipital processes with 

 each other dorsal to the notochord, but their ventral margins 

 are separated by the notochordal sheath (fig. 8). A canal 

 extends ventroposteriorly from the floor of the otic capsule and 

 opens to the exterior posteroventral to its wall. The mesial 

 wall of this canal is formed by the occipital cartilage (fig. 8). 

 The canal contains a part of the sacculus and the fibers of the 

 glossopharyngeal and vagus nerves. A common canal for these 

 two cranial nerves is also found in the 6.6-mm. Gasterosteus, 

 so it may be stated that Syngnathus is more precocious in devel- 

 opment in this part of the cranium than in the more anterior 

 parts, since all of the parts so far noted have been comparable 

 in state of development to the same parts in the 5.7-mm. 

 Gasterosteus. 



The occipital masses lateral to the notochord persist for a 

 short distance posteriorly, but do not meet dorsal to the brain 

 to form an occipital arch (figs. 2, 8). They gradually diminish 

 in extent, the ventrolateral parts disappearing first, as would 

 be expected, since these represent the posterior ends of the 

 parachordals. Finally, the cartilage on the dorsolateral surfaces 



