LECT. VII.] THE BASI-CRANIAL BEAM. 179 



post-squamosal process. Thus in the dry skull of the 

 adult we see the meatus or ear-passage as a thick-lipped 

 chink, the small opening being vertical ; its walls are the 

 inwedged outgrowth of the annulus, or ear-ring. All 

 these parts are so completely fused together that nothing 

 but the foetal or embryo skull will show how far each 

 bony "centre" extended. There are 110 basi-sphenoidal 

 excavations, or air cavities, in the base of the skull, as in 

 the Hedgehog and Tenrec. 



The ossicula auditus, or small bones of the ear-chain, 

 are formed and finished very early, but they show no 

 leanings towards those of the Marsupials. 



There is one thing, however, in this skull which is 

 more important to the developmentalist than all the 

 others put together. During the many years in which 

 I have worked at this subject, there is one bone which 

 I have come across, again and again, but always with 

 renewed interest. The name of this bone the " para- 

 sphenoid" is not found in treatises on human ana- 

 tomy. It is the first bone in the vertebrate series 

 taken from the more superficial structures to support 

 the deep cartilaginous endo-skeleton, and the first to 

 appear in the embryos of Fishes and Amphibia. To one 

 familiar with the Frog's skull, this is a striking part of 

 that instructive piece of cranial architecture, which is 

 intermediate between that of the lower and the higher 

 tribes. Under the Frog's basis-cranii there is a 

 dagger-shaped bone, this is the parasphenoid; it sup- 



