IX.] THE INVESTING MASS. 227 



4. The investing mass (Fig. 68, iv) is on the fourth day 

 a broad plate of tissue ensheathing the notochord and arching 

 upwards on each side and especially behind. Laterally it 

 encloses the auditory sacs, and the tissue surrounding these 

 (forming the so-called 'periotic capsules') is in the chick never 

 separate from the investing mass. In front it becomes 

 narrowed, and at the same time excavated so as to form 

 a notch on each side (Fig. 68, 5) through which the fifth 

 nerve passes ; and in front of this again it becomes expanded. 



In order to render our subsequent account more intelligi- 

 ble, we may shortly anticipate the fate of the investing mass. 

 Behind it grows upwards, and the two outgrowths meet 

 above so as completely to enclose the medulla oblongata, 

 and to circumscribe a hole known as the 'occipital foramen! 

 And it is at this point only that the roof of the skull is at 

 any period formed of cartilage. 



Later than this there appear in the investing mass a 

 series of ossifications forming the whole of the occipital bone, 

 and the skeleton of the ear. Knowing this, we shall be able 

 to speak of the different parts of the investing mass, as the 

 regions of the various bones 'ex-occipital,' 'basi-occipital' etc. 

 even before the ossifications, which mark these out, have 

 appeared. 



5. In considering the paired rods of cartilage attached to 

 the investing mass, we will commence with the most an- 

 terior. These are known as the 'trabeculce cranii (Fig. 68, tr). 

 They are two narrow rods whose proximal ends are attached 

 to the front end of the investing mass, of which they appear 

 like a forward continuation, while distally they unite with 

 each other. There is thus left an oval (or more nearly 

 circular) space where the cartilage forming the base of the 

 skull is deficient. This space is enclosed behind by the in- 

 vesting mass, and on the two sides and in front by the trabe- 

 cular; it is called the 'pituitary space' (Fig. 68, pts), and in 

 it lies the 'pituitary body.' 



Where the trabecular unite in front, they form a some- 

 what expanded plate of cartilage continued anteriorly into 

 two diverging horns, which subsequently develope into the 

 ali-nasal cartilages. Owing to the cranial flexure the trabe- 

 cular at first lie in a different plane from the investing mass. 



In function, the trabecular seem almost to serve as a con- 



15—2 



