SKULL MAMMALS 



183 



It is unfortunate that we have no adequate accounts of the earliest stages of 

 the cartilage skull. The chondrocranium arises in pre- and postchordal parts. 

 The basal plate is formed by the parachordals with the addition of three occipital 

 vertebrce as in all Amniotes, so far as known, these fusing early with the para- 

 chordals, a basicranial fenestra occurring occasionally. The jugular foramen is 

 formed as elsewhere, between the first occipital vertebra (postotic pillar) and the 

 otic capsule and is closed dorsally by growth from these. Dorsally each half 

 of the postotic region continues into a broad plate dorsal to the foramen magnum, 

 and is at first distinct from the synotic tectum which arises as a discrete cartilage. 



The otic capsules, from the first, lie in the cranial floor, each having its major 

 axis inclined medially inwards and forwards. Sometimes they arise apart from 

 the basal plate to which they become attached in various ways in different 



Fig. 195. — Chondrocranium of 14 mm. human embryo (Fawcett, '19). d, dens 

 epistrophei; h, hypophysis; n, notochord; oc, otic capsule; p, parachordal plate; s, stalk 

 of hypophysis; /, trabecula; 12, hypoglossal canal. 



mammals. At first floor and roof of the capsules are separate. A ('parietal') 

 plate of cartilage arises from the upper posterior part of each capsule and passes 

 behind into the tectum, and is usually connected in front by an orbito-parietal 

 commissure with the orbitosphenoid, and thus is a marginal taenia. 



It is uncertain whether the trabecule arise separately from the basal plate; 

 if they do, they unite early. The bars and the prehypophysial common tra- 

 becula are short and the hypophysial fenestra is nearly closed, there being for a 

 short time a narrow opening for the hypophysial stalk. A small sphenolateral 

 cartilage (ala temporalis — sometimes a separate chondrification) passes laterally 

 from the side of the trabecula, with a wide gap between it and the otic capsule, 

 and bears on its lower posterior border a small, ventrally directed process which 

 later forms the lateral lamella of the pterygoid process of the sphenoid bone. 



The third branch of the fifth nerve passes through the gap (foramen lacerum) 

 between sphenolateral (alisphenoid) and otic capsule, but is soon surrounded 

 by cartilage which connects with the alisphenoid, thus outlining the foramen 

 ovale. The other branches of the trigeminus pass forwards, medial to the ali- 

 sphenoid, and then out, at first through the orbital fissure. Later they are 

 enclosed in a foramen rotundum, formed in the same way as the f. ovale. 



