626 JOHN D. KERNAN, JR. 



cranial, lateral and caudal parts of the orbital wings have not 

 begun to develop. (3) Almost the entire absence of the turbi- 

 nates, which Jacoby finds beginning to form. Levi mentions 

 them as being already cartilaginous in his 28 mm. embryo. (4) 

 There are, in my embryo, no external pterygoid processes, which 

 Jacoby found in his. 



In the sphenoid region, my embryo seems, in certain respects, 

 more developed, as it shows a posterior clinoid process, a foramen 

 rotundum, and incomplete foramen ovale. 



As compared to the fully developed chondrocranium, my 

 model lacks (1) in the nasal region, the nasal roof, the lamina 

 cribrosa, the connection between ala orbitalis and sphenoid, 

 and the turbinates. (2) In the sphenoid region there is no 

 optic foramen; the alae orbitales are poorly developed and are 

 not connected with the sphenoid body; the pterygoid is lacking. 

 (3) In the otic region, the tegmen tympani has only just begun 

 to form. (4) In the occipital region, the disproportionate size 

 of the foramen magnum at once attracts attention, due to the 

 occipital wings not having met in the midline. The paracondy- 

 loid processes are disproportionately large; in later stages they 

 undergo absorption. The fact that they are not yet entirely 

 fused with the occipital squama should be particularly noted. 



As to the light which this reconstruction throws on the course 

 of the development of the skull, this may be said: The basal plate 

 first fully develops. The occipital squamae develop and unite 

 about the foramen magnum, first through the slender band of 

 cartilage which connects the partes canaliculares and fuses with 

 the squama by its caudal edge. 



The development of the otic capsules and sphenoid body takes 

 place about the same time, and the otic capsules then fuse to 

 basal plate and occipital squamae. The orbital and temporal 

 wings of the sphenoid region next appear, and a little after them 

 the nasal septum and lateral nasal parts. This is about the 

 stage of my embryo. 



The next step (Levi) is the elevation of the ventral end of the 

 basal plate, involving rotation also of the otic capsules through 

 almost one-quarter of a circle. This causes the ventral ends of 



