178 THE FIFTH DAY. [CHAP. 



portion of this investing mass sends upwards along the sides 

 of the brain two lateral projections or wings, which enclose 

 the rudiments of the internal ear. In the chick the portions 

 which thus inclose the auditory sacs seem never to be at any 

 time separate from the remainder of the investing mass. At 

 the front end of the notochord the cartilaginous investing 

 mass divides into two horizontal branches in the form of two 

 cartilaginous rods called the trabecules (Fig. 55, tr.), which 

 passing forward (in a somewhat different plane from the 

 investing mass), meet again in front, and so enclose a space 

 called the pituitary space pts, into which the infundibulum 

 extends downward. In front of this junction, the trabecule 

 expand into a somewhat broad plate (subsequently developed 

 into the ethmoid and nasal cartilages), which ends in two 

 horns in the interior of the fronto-nasal process. 



The front end of the notochord probably defines the 

 anterior boundary of the basi-occipital. At first it extends 

 qu^fce up to the pituitary space and the starting-point of the 

 trabecular. Subsequently, however, there takes place be- 

 tween it and the pituitary space a growth of cartilage in 

 which the ossification for the basi-sphenoid takes place. 



The lateral projections at the hinder end of the investing 

 mass grow up behind, and completely enclose that part of 

 the neural canal from which the medulla oblongata is de- 

 veloped, and in it ossifications arise to form the occipital 

 bones and the bones which invest the auditory labyrinth. 



It is important to notice that the only segment of the 

 skull, which primarily forms a cartilaginous roof to any part 

 of the brain, is the occipital segment. The roof of the re- 

 mainder of the skull is formed by membrane-bones. 



For the histological differences observable in the develop- 

 ment of cartilage and membrane bones, we must refer 

 the reader to treatises on histology; for our purpose it is 

 sufficient to say that a membrane-bone is one which is not 

 preformed in cartilage, while a cartilage-bone is one in which 

 the ossification takes place in a bed of cartilage, which fills 

 the place subsequently occupied by the bone. 



The trabecular together with the cartilage between the 

 pituitary space and the end of the notochord give rise to the 

 sphenoid bone, while in the cartilage in front of the trabecular 

 the ethmoid and nasal bones are formed. 



