VII.] THE INVESTING MASS. 177 



3. As we mentioned in the last chapter, the formation 

 of the primitive cranium commenced upon the fourth day. 

 This in its earliest stage, inasmuch as it is composed of con- 

 densed but otherwise only slightly differentiated mesoblast, 

 may be spoken of as the membranous cranium. 



On the sixth day, true hyaline cartilage makes its ap- 

 pearance ; and the primitive membranous cranium gives 

 place to the primitive cartilaginous cranium. 



The cartilage which is the first to appear, forms a thick 

 plate called the investing mass of Rathke (Fig. 55, iv.), sur- 

 rounding the whole of that portion of the notochord which 

 projects in front of the foremost pi-otovertebra. The hinder 



Pig. 55. 



View from above or the Investing Mass and of the Trabecule on 

 THE Fourth Day of Incubation. (From Parker.) 



In order to shew this, the whole of the upper portion of the head has been 

 sliced away. The cartilaginous portions of the skull are marked with the dark 

 horizontal shading. 

 cv. I. cerebral vesicles (sliced off), e. eye. nc. notochord. iv. investing mass. 



9. foramen for the exit of the ninth nerve, cl. cochlea, hsc. horizontal 



semicircular canal, q. quadrate. 5. notch for the passage of the fifth nerve. 



Ig. expanded anterior end of the investing mass. pts. pituit try space. 



tr. trabeculae. The reference line tr. has been accidentally made to end a 



little short of the cartilage. 



E. 12 



