THE SKULL. 



565 



of very interesting memoirs have been published on the subject 

 by Parker, which together constitute a most striking contribution 

 to our knowledge of the ontogeny of the skull in a series of 

 types ; and in Germany Gegenbaur's monograph on the cephalic 

 skeleton of Elasmobranchii has greatly promoted a scientific 

 appreciation of the nature of the skull. 



In the present chapter only the most important features in 

 the development of the skull will be touched on. 



It will be convenient to describe, in the first instance, the 

 development of the cartilaginous elements of the skull. 



The Cranium. The brain is at first enveloped in a contin- 

 uous layer of mesoblast known as the membranous cranium, 

 into the base of which the an- 

 terior part of the notochord is 

 prolonged for some distance. 

 The primitive cartilaginous 

 cranium is formed by a differ- 

 entiation within the membra- 

 nous cranium, and is always 

 composed of the following parts 



(fig- 323) : 



(1) A pair of cartilaginous 

 plates on each side of the 

 cephalic section of the noto- 

 chord, known as the parachor- 

 dals (pa. ck}. These plates to- 

 gether with the notochord (nc) 

 enclosed between them form a 

 floor for the hind- and mid- 

 brain. The continuous plate, 

 formed by them and the noto- 

 chord, is known as the basil ar 

 plate. 



(2) A pair of bars forming 

 the floor for the fore-brain, 



iff 



Cl? 



pa.ch. 



CIA 



FIG. 323. HEAD OK EMBRYO DOG- 

 FISH, SECOND STAGE ; BASAL VIEW OF 

 CRANIUM FROM ABOVE, THE CONTENTS 

 HAVING BEEN REMOVED. (From 



Parker.) 



ol. olfactory sacs ; an. auditory capsule; 

 nc. notochord; py. pituitary body ; pa.ch. 

 parachordal cartilage ; tr. trabecula ; inf. 

 infundibulum ; C.ir. cornua trabeculse ; 

 pn. prenasal element ; sp. spiracular cleft ; 

 br. external branchiae; Cl. 2, 4. visceral 

 clefts. 



known as the trabeculae (tr). 

 These bars are continued forward from the parachordals. They 

 meet behind and embrace the front end of the notochord ; and 

 after separating for some distance bend in again in such a way 



