414 INTRODUCTION TO CRANIATA [CH. XIX 



as the cheesy substance is called. These concentric layers make up 

 what is called the capsule of the cartilage cell. When a cartilage 

 cell divides, its daughter cells proceed to form new capsules within 

 the old one and so the main mass of the cartilage is gradually built 

 up. In bone, on the other hand, the cells remain single while the 

 ground substance becomes hardened by depositions of phosphate 

 and carbonate of lime. The spaces occupied by the cells are known 

 as lacunae, and the delicate processes which connect the cells give 

 rise to the capillary canals known as canaliculi in the dried bone, 

 whilst the spaces occupied by blood-vessels traversing the bone are 

 known as Haversian canals. 



In the simplest form the skull consists of two pairs of pieces of 



cartilage, one pair of which embrace the front end 



skuii. mltlva of the notochord and are termed the parachordals. 



In front of these is the second pair, termed the 



trabeculae, which are united behind and before with each other 



but which diverge in the middle so as to embrace between them 



the pituitary body, an appendage of the brain. The parachordals 



develop ridges which wall in the sides of the brain and may form 



a roof over its hinder portion. 



The brain is only the enlarged and modified anterior end of the 

 neural tube, and the existence of a skull-roof is correlated with the 

 presence of neural arches protecting the hinder part of the 

 nervous system. These arches consist of paired pieces of cartilage 

 meeting above the neural tube. They have been shown to be formed 

 as differentiations of solid cellular outgrowths of the myotomes which 

 represent the hollow sclerotomes of Amphioxus, and hence it may be 

 that the parachordal part of the cranium at any rate is derived from 

 outgrowths from the walls of the most anterior myotomes which 

 early become fused with one another and otherwise modified. 



Haemal arches, which are paired pieces of cartilages with their 

 upper ends implanted in the sheath of the notochord and their outer 

 ends directed downwards, are also always present, and like the neural 

 arches are derivatives of the myotomes. In the region of the tail 

 the haemal arches meet each other so as to form a V beneath the 

 notochord, but in the trunk they simply project out between adjacent 

 myotomes as par apophyses, the ends of which may become movable 

 on the basal parts and are then known as ribs. 



The brain of all Craniata is sharply divisible into three primary 



regions called fore-brain, mid-brain and hind- 



Bram * brain (Fig. 207). Of these the first is certainly 



