296 EGBERT J. TERRY 



In sections of 12 mm. embryos (fig. 17) the notochord, as it 

 enters upon its cranial course, lies at first dorsad of the still 

 imperfect basal plate, then sinks into the otic portion of the 

 plate; it is now surrounded on all sides by cartilage at the level 

 of the primary parachordal commissure. The notochord emerges 

 from the basal plate at its free anterior end-, which is inclined 

 dorso-cephalad, and terminates in the mass of mesenchyma 

 which fills the interval (fenestra basicranialis posterior) between 

 the expanded end of the basal plate and the hypophyseal car- 

 tilage. Where the notochord enters the cranium it is surrounded, 

 as in the preceding stage, by a layer of mesenchyma which ex- 

 tends caudally to join with the chondrifying tissue forming the 

 centrum of the atlas. The centrum stands in the same trans- 

 verse plane as the arches of the atlas, from which it is separated' 

 by a stratum of less compact mesenchyma. Sagittal sections 

 show a plane of densely packed nuclei separating the centrum 

 of the atlas from the centrum of the epistropheus already laid 

 down in young cartilage. Chondrification of the hypochordal 

 arch of the atlas is less advanced than that of the tissue be- 

 neath the notochord between the caudal margins of the para- 

 chordal plates; in the latter young cartilage is present, while 

 in the hypochordal arch of the atlas mesenchyma alone is to be 

 found. 



Between the caudal margin of the parachordal plates and the 

 lateral mass of the atlas is a transverse stretch of deeply staining 

 tissue, dense laterally and thin medially where it meets the 

 mesenchyma about the notochord. Its later history is not clear. 



Two marked advances in development of the floor of the cra- 

 nium appear in a van Wijhe preparation of 15 mm. (fig. 7). 

 First, the basal plate is nearly perfected, the unchondrified part 

 in the occipital region being now considerably reduced. The 

 bar completing the boundaries of the hypoglossal foramen stands 

 opposite the primary commissure of the parachordals and is 

 thickened at its origin, just where the jugular tubercle and 

 paracondyloid processes later are developed. At the ^ame spot a 

 slight projection forward toward the cochlear capsules, medial 

 to the vena jugularis, is probably the beginning of the basi- 



