DEVELOPMENT OF THE SKULL OF EMYS 747 



rate cartilaginous center at some distance from the corpus^ and is 

 surrounded by a mass of chondroblasts which is continuous with 

 it. 



The processus HnguaUs arises from a pair of short processes 

 extending forward from the front margin of the corpus hyale at 

 a short distance from each other and converging sHghtly toward 

 their distal ends. In the stage modelled it is present as a single 

 median rod of cartilage with only a shght indication on its dorsal 

 surface, in the form of a shallow longitudinal groove, of its paired 

 origin. Cross sections, however, prove its double nature even 

 at a much later stage. 



MEMBRANE BONES 



The membrane bones are all laid down in the stage modelled 

 with the exception of the parasphenoideum, quadrato-jugale, and 

 the complementare of the lower jaw. 



The squamosum (s., figs. 24, 25 and 26) is a thin plate of bone 

 of irregular triangular form, exhibiting a convex lateral and a 

 concave median face. It Ues parallel to the outer side of the 

 quadratum, overlying the posterior extension of the same, and 

 separated from it by only a narrow layer of connective tissue. 

 Its posterior margin projects slightly beyond the quadratum 

 while its dorsal margin shows a tendency to curve medially to 

 embrace the quadratum- more closely. The ventral angle of the 

 bone is also bent medially below the posterior end of the quad- 

 ratum. 



As the embryo increases in size the squamosum comes to he 

 with its postero-ventral angle resting against the crista parotica 

 while its dorsal margin extends medially above the quadratum 

 to come in contact with the lateral wall of the otic capsule dorsal 

 to the prominentiae canalis semicircularis lateraUs and ampul- 

 laris posterior. The anterior angle of the squamosum also comes 

 to extend relatively further forward in older embryos. 



The first appearance of a squamosum is in an embryo having 

 a carapace length of 7 mm., at which stage it has the form of 

 a shallow saucer in contact by its margins with the posterior 

 extension of the quadratum. 



