CHONDROCRANIUM OF A 20 MM. HUMAN EMBRYO 615 



manner of chondrification explains the appearance of the model 

 at this stage, the thick condyles laterally, united ventrally in a 

 ridge but separated dorsally by a deep groove (fig. 6). 



The atlas similarly begins to chondrify in its lateral masses, 

 which later unite ventrally to form the anterior arch (figs. 8 and 

 9, B). 



In calf embryos Froriep found absorption of the Iiypochordal 

 arches, the basiventrals, and interpreted the median region of 

 the occipital as derived from the centra of occipital vertebrae. 

 DeBurlet, in Balaenoptera rostrata, has shown the caudal end 

 of the occipital region to be suprachordal in position, not hypo- 

 chordal, a condition which plainly excludes the possibility of 

 homologizing this region with basiventrals. On the other hand 

 Weiss ('01), working with rat embryos, Noordenbos ('05), with 

 Talpa, and Gaupp with rabbit and opossum embryos, have all 

 shown that in these animals the basioccipital is formed by hypo- 

 chordal elements. It is evident that we are in need of further 

 investigation of this region. 



My embryo shows the oldest cartilage just mesial to the hypo- 

 glossal canals, and in the arches close to the outer side of the 

 canals. There is no sign of bone formation. 



The occipital wing shows on its outer surface, lateral to the 

 edge of the foramen magnum, the thin area of cartilage known as 

 the paraforminal area (Macklin) (figs. 2, 3). There is no sign 

 of the lateral occipital protuberance. On the ental surface the 

 groove between occipital wing and parietal plate, the occipito- 

 parietal groove (Macklin) may be traced, but there is no occip- 

 ito-parietal fissure (fig. 1). The internal capsulo-occipital 

 groove is well marked; the external, only fairly well (figs. 1, 2, 3). 



The otic region is composed of the otic portion of the basal 

 plate, the paired otic capsules, and the paired parietal plates. 



The otic portion of the basal plate (figs. 1 and 3) is thick and 

 wide caudally where it joins the occipital region. This juncture 

 is caudal to the site of that primitive break in the cartilage al- 

 ready described. The plate here is horizontal. Cranially the 

 dorsal surface rises rather sharply, becomes convex, and passes 

 without demarcation into the steeply sloping dorso-caudal sur- 



