444 JAMES ERNEST KINDEED 



THE CHONDROCRANIUM OF THE 12-MM. STAGE 

 A . The neurocranium 



The anterior end of the ethmoid plate of the 12-nim. Syngna- 

 thus is much broader and blunter than it is in the neurocranium 

 of the 8-mm. stage. Its relations to the palatine and rostral 

 cartilages are similar (fig. 11). The anterior end, however, is 

 no longer turned dorsally, but has straightened out, carrying 

 with it the attached elements. Posterior to the broad rostral 

 process, the ethmoid plate gradually diminishes in size. It 

 becomes wedge-shaped at first and then more triangular in cross- 

 section. The dorsal part of the ethmoid is the thicker. 



The vomer primordium represented in the 8-mm. stage by a 

 mass of cells ventral to the ethmoid plate is now an osseous 

 lamella embedded in a mass of osteogenetic cells in the same 

 region (fig. 12). 



The dorsal surface of the ethmoid plate is surmounted by a 

 septum of fibrous-connective tissue. The relationship of this 

 to the cartilage is the same as that of the osseous ridge which 

 later develops in this place. This ridge of connective tissue is 

 connected with the stroma enclosing the olfactory pits and is 

 continuous posteriorly with the membranous interorbital septum. 

 Histologically, the ethmoid cartilage is the "same as it is in the 

 8-mm. stage except for an increase in size and the presence of a 

 thicker perichondrium. 



The ectethmoid cartilages, which in the 8-mm. embryo were 

 separate, connected with each other by connective tissue only, 

 have now fused with each other dorsomesially to form a horse- 

 shoe-shaped mass of cartilage posterior to the olfactory pits 

 (fig. 12). The posterior margin of this mass forms the anterior 

 boundary of the orbits. Thfe anterior end of the brain lies in a 

 trough on the dorsum of this ectethmoid arch, the margins of the 

 trough projecting posteriorly for a short distance in the mem- 

 branous cranial wall (figs. 10, 11). The olfactory nerves, after 

 leaving the olfactory pits, pass to the mesial margins of the sides 

 of the arch and extend posteriorly within it (fig. 12). Thus at 

 this stage the olfactory nerves have not been separated from 



