312 EMBEYOLOGY" OF THE LOWEE VEETEBEATES oh. 



In the ease of the young Protopterus the cartilage goes on 

 extending considerably with growth. In the region of the auditory 

 capsule it spreads dorsalwards and reaches the middle line so as 

 completely to roof in the cranial cavity at this level. Further 

 forwards in the region of the Gasserian recess the side wall of the 

 cranium also extends dorsalwards, though in this region there 

 remains a wide deficiency in the cartilaginous roof. 



In Lepidosiren the increase in cartilage is less marked, in fact 

 the chondrocranium of the adult remains in many respects in the 

 same condition as that of the larval Protopterus. 



The Dipnoan chondrocranium obviously belongs to that type in 

 which the primary basal cartilages of the skull are continuous on each 

 side — almost or quite from the beginning, the separation into distinct 

 trabecular and parachordal portions, if visible at all, being confined 

 to a very brief period, and in which the cranial cavity extends 

 forwards between the orbits (platybasic type). Seeing that a similar 

 type of chondrocranium occurs in the majority of the holoblastic 

 lower Vertebrates the probability is that it represents a more nearly 

 primitive condition than the tropibasic type, with parachordal separate 

 from trabecula, which is more usual in the meroblastic vertebrates. 



Development of the Chondrocranium in Elasmobranchs. — 

 As the chondrocranium has for its main function the support and 

 protection of the brain, and develops in close relation with it, it will 

 be of interest to compare with the development of the Dipnoan cranium 

 that of one of the meroblastic Vertebrates in which the brain is modified 

 in early stages by a greatly developed cerebral flexure. The Elasmo- 

 branchs are admittedly the most nearly primitive of such forms and 

 • may therefore most suitably be taken as the example. 



Here (Pristiurus and Acanthias — Sewertzoff, 1899) the first indica- 

 tions of skull development make their appearance about stage 25 

 (see Chap. XL), as a concentration of mesenchyme on each side of the 

 notochord about the level of the otocyst. This spreads, headwards 

 and tailwards, as a parachordal strand of prochondral tissue, extend- 

 ing anteriorly as far as the Facial nerve and continuous posteriorly 

 with the rudiment of the vertebral column. As the parachordal plate 

 takes definite form it develops in its occipital' portion segmentally 

 arranged rounded swellings which project dorsally between the nerve- 

 roots and correspond in position with the intermuscular septa. 



The prochondral parachordal plates gradually become chondrified 

 and at the same time they extend outwards and dorsalwards to form 

 the basilar plate. As they do so the metameric projections flatten 

 out and disappear in the anterior portion while posteriorly they become 

 more pronounced, growing up dorsally between the nerve-roots and 

 finally meeting over the roots so as to enclose them in distinct foramina. 

 In the region behind the definitive skull the swellings in question do not 

 fuse but develop into discrete arch-elements. The last of the swellings 

 to be included in the skull would appear to be, as a rule, in Sharks 

 and Dog-fish that between metotic myotomes 7 and 8 (Braus, 1899) 



