210 OUTLINES OF CHORDATE DEVELOPMENT 



2. The Skull 



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The fully formed skull is a complex organ formed by the 

 association, and more or less extensive fusion, of several diverse 

 elements; these are (a) the cranium, (6) the sense capsules, (c) 

 the visceral arches (in part), (d) the notochord (in part), (e) 

 vertebral elements, (/) membrane or derm bones. Before pro- 

 ceeding to describe the formation and association of these ele- 

 ments in the frog, we should note that here no embryologically 

 distinct vertebral elements are included in the skull; their 

 inclusion in the above list is based upon phyletic and compara- 

 tive grounds, and upon the behavior of the anterior somites. 



A. THE CRANIUM AND SENSE CAPSULES 



In tadpoles of about 7 mm. the first rudiments of the cranium 

 are formed as a pair of curved strands of dense tissue, soon 

 becoming cartilaginous, along the ventro-lateral surfaces of the 

 fore-brain. These are the rudiments of the trabeculce or tra- 

 becular cartilages (Fig. 80, A). They rapidly extend forward 

 and fuse across the mid-line between the olfactory organs, 

 forming there the rudiment of the internasal plate; each rod then 

 continues forward as the trabecular cornu, which expands 

 slightly, partly enclosing the olfactory organ and forming the 

 olfactory capsule. In front of the olfactory capsule the trabeculre 

 unite with the rudiments of a pair of labial or suprarostral 

 cartilages, lying in the extended upper lip. Posteriorly the 

 trabeculae extend beneath the mid-brain, embracing between 

 their ends the anterior extremity of the notochord, which it 

 will be recalled extends forward to the mid-brain region. Soon 

 similar tissue thickenings extend posteriorly each side of the 

 notochord in the hind- brain region; these are the indications 

 of the parachordce or parachordal cartilages. The rudiments of 

 the parachordals now fuse with the posterior ends of the tra- 

 becula?, enclosing the tip of the chorda and forming a continuous 

 plate beneath the hind-brain, known as the parachordal plate 

 (Fig. 80, A}. 



