PRIMORDIAL CRANIUM OP THE CAT 295 



it by a transverse plane of unstained tissue is the broad conical 

 mass of young cartilage surrounding the notochord, the begin- 

 ning of the atlantal centrum. Between the apex of the latter 

 and the commissure of the parachordals the notochord stands 

 free of any chondrified tissue as evidenced by the absence of the 

 blue stain about it. 



Transverse sections of an embryo of 10.6 mm. (fig. 15) make 

 clear the relations of the notochord, dens epistrophei and para- 

 chordal plates at the stage of cartilaginous structure of the 

 latter. The parachordals are connected across the median plane, 

 ventrad of the notochord by loose tissue. This tissue presents 

 more and more the characters of mesenchyma when traced 

 toward the median plane, and more and more the condition of 

 cartilage when followed toward the parachordals into which it 

 passes. An imperfect basal plate is thus formed in the occipital 

 region which extends from side to side ventrad of the notochord 

 and which passes laterally into the lateral occipital arches. 

 Figure 15 shows the notochord covered dorsally and laterally 

 by a layer of cellular tissue lying upon the dorsal surface of the 

 stretch of me.senchyma connecting the parachordal plates. 

 When followed caudad this layer increases in thickness, especially 

 on the dorsal side of the notochord, and passes over into the 

 mesenchyma of the centrum of the atlas. 



In van Wijhe preparations of 12 mm. (fig. 6) the most impor- 

 tant differences from the preceding stage in the posterior part 

 of the cranium are the presence of a hypoglossal foramen, the 

 beginning of the parietal plate and the slender cartilaginous basal 

 plate of the otic region. The hypoglossal foramen has resulted 

 from the development of a bar of cartilage uniting the lateral 

 angle of the parachordal plate and lateral occipital arch outside 

 the roots of the twelfth nerve. The beginning of the parietal 

 plate appears standing free, dorsad of the posterior semicircular 

 canal of the otic capsule. The basal plate of the otic region is 

 continuous behind with the primary commissure of the para- 

 chordals, terminates in front in an expansion which extends 

 nearly as far as the cochlear wall, and anteriorly projects some- 

 what dorsad of the hypophyseal cartilage. 



