202 DEVELOPMENT OF THE MARSUPIAL SKULL, 



The preraaxillary bone is partly ossified. Ifc forms a support 

 for the germs of the incisor teeth, and a floor for the anterior 

 part of the nasal capsule. In the cleft between the nasal capsules 

 in front there are indications of an ascending internasal process, 

 as in reptiles. The process is not ossified, but the strand of 

 differentiated cells, perhaps degenerate osteoblasts, can be easily 

 traced into the region to be occupied by the nasal bone. 



The maxilla is a triangular-shaped bone when viewed from 

 below. It is most developed in the region of the teeth-germs, 

 but the secondary palate is well formed. The roof of the mouth 

 is markedly concave. 



The palatine is well ossified and forms the walls of the pos- 

 terior nares. The portion which forms the secondar}^ palate is 

 the larger. 



The pter3'goid is still small, and only partly ossified. It rests 

 on the lower part of the anterior end of the alisphenoid cartilage. 



The jugal, the lachrjmial, and the squamosal are beginning to 

 ossify, but too imperfectly to be drawn. 



The mandible is well ossified and of large size. At its posterior 

 end the coronoid process and the angle are already typically 

 formed, and the articular condyle is made up of cells somewhat 

 like imperfectly formed cartilage-cells, but there is no distinct 

 cartilage. 



The following series of sections will illustrate the relationships 

 of the more important structures. Section A is from the unborn 

 embryo described and figured as embryo F. All the other 

 sections are from the newly born embryo described and figured 

 as embrj^o H. 



Section A(Pl.xiii., fig. 13). This section has been figured from the 

 unborn specimen as, owing to the different staining, the cells of 

 the developing premaxilla are better seen. The section is through 

 the front part of the anterior nares. Part of the capsul^ is seen 

 as a half ring, which is a short distance apart from that of the 

 opposite side. Between the two cartilages are seen the strands 

 of cells which represent the ascending internasal processes of the 

 premaxillfe. 



