THE THEORY OF THE VERTEBRATE SKULL. 291 



tends backwards, as far as the end of the body ; forwards, only 

 to the interspace between the auditory capsules. 



"(2.) The gelatinous investing mass, which, at first, seems 

 only to constitute a band to the right and to the left of the 

 notochord, forms around it, in the further course of development, 

 a sheath, which ends in a point posteriorly. Anteriorly, it sends 

 out two processes which underlie the lateral parts of the skull, 

 but very soon coalesce for a longer or shorter distance. Pos- 

 teriorly, the sheath* projects but little beyond the notochord ; 

 but, anteriorly, for a considerable distance, as far as the infun- 

 dibulum. It sends upwards two plates, which embrace the 

 future central parts of the nervous system laterally, probably 

 throughout their entire length. 



" (3.) The investing mass of the notochord is the material 

 out of Avhich the vertebral column and a great part of the skull, 

 though not the whole skull, are developed. 



" (4.) The most essential part of a vertebra is its body. 

 With the exception of a few cartilaginous fishes, the cartilaginous 

 foundation of that body (the notochord having disappeared ear- 

 lier or later), has the form of either a ring, or a half ring ; or, 

 as is the case among the Mammalia, forms a solid mass, having 

 the form of the segment of a cylinder. Subordinate parts of 

 the vertebra are the vertebral arches and transverse processes, 

 together with the ribs, which all, at the time they take on a car- 

 tilaginous character, appear as rays of the body, though some- 

 times they are not developed at all. Only in rare cases (Petro- 

 myzon) are vertebral arches developed without vertebral bodies ; 

 that part of the investing mass of the notochord which is, in 

 other cases, applied to the formation of such bodies, acquiring 

 only a membranous consistency. 



" (5.) From that part of the investing mass of the cephalic 

 part of the notochord, which consists of the anterior part of the 

 sheath of the notochord and its anterior paired processes, are 

 developed the basi-occipital, the basi-sphenoid, and the ethmoid, 

 so that the ethmoid is the most anterior of the parts of the 

 skeleton which take their origin from the investing mass of the 

 notochord. The basi-occipital is formed in that part of this 



* Perhaps with rare exceptions, as in Fistularia tabaccaria. 



v 2 



