328 CHARLES CLIFFORD MACKLIN 



mentioned. The similarity in general outline is quite striking, 

 when these are compared with my specimen. The illustration 

 of the caudal aspect of the skull of Macacus shows a flattened 

 condition of the partes cochleares closely analogous to that which 

 obtains in my model. Though the dorsal part of the cranium is 

 relatively shorter it is very suggestive of the cranium of homo, 

 especially from the rudimentary condition of the side-walls. 



CHONDROCRANIUM 



Planum basale 



That portion of the central stem of the chondrocranium which 

 forms its dorso-caudal limb and is traversed medially and longi- 

 tudinally by the notochord, is known as the planum basale 

 (figs. 1, 2 and 5). It is an elongated and unperf orated plate of 

 cartilage of varying thickness, which extends from the inter- 

 condylar incisure of the foramen magnum to the dorsal border 

 of the hypophyseal fossa, and forms the most dorsal part of the 

 skull-base. Its cartilaginous substance is directly continu- 

 ous with that of three regions; with the occipital region caudo- 

 dorsally where the planum is seen to pass over into the lateral 

 portions of the occipital anlage, with the orbitotemporal region 

 cranio-ventrally, where it coalesces with the body of the sphe- 

 noidal cartilage at the dorsum sellae, and with the otic region 

 laterally. The entire lateral border, with the exception of the 

 extreme upper portion, is united to the pars cochlearis of the otic 

 capsule, the line of union being indicated upon the medial sur- 

 face of the latter by an elongated, narrow, crescentic strip (fig. 7). 

 This line of union is formed of cartilage throughout, and in its 

 cranial two-thirds this is of the same character as that of the 

 adjoining parts. In the lower third, however, there is to be 

 seen, microscopically, a distinct but thin sheet of younger cartil- 

 age, separating the adjacent parts of the planum and the cochlea, 

 this being the last indication of the primitive separation of these 

 parts. 



It is worthy of note in passing that the 17 mm. stage of Levi 

 shows the first indication of a union in the human skull between 



