CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 305 



Both in the embryo and in the adult each exoccipital arises from the side of 

 the posterior half of the basioccipital and is perforated at its origin by the hypo- 

 glossal canal. In this embryo the right hypoglossal canal is divided on the cranial 

 side into two parts by a cartilaginous bar; on the pharyngeal side there is but a 

 single orifice, a condition not uncommon in the adult. The axis of the hypo- 

 glossal canal is nearly dorso-ventral in the chondrocranium, while in the adult 

 it passes almost laterally. 



The lateral borders of the foramen magnum are formed by the lateral parts 

 of the occipital or exoccipitals and the squamae. There is a general similarity in 

 direction, in position, and in form of the exoccipital in the embryo and adult. 

 Included in the lateral occipital and constituting much of it, as seen in this view, 

 is the occipital neural arch, which is partially separated near its dorsal end from 

 the exoccipital and squama by a fissure (the occipital fissure) which is filled with 

 condensed mesenchyme. 



That part of the exoccipital (the alar lamina) which continues upward from the 

 jugular process is concave on its inner surface for the large transverse sinus. The 

 sinus continues upward and outward, as in the adult, across the exoccipital and 

 mastoid portion of the temporal, but turns forward over the dorsal margin of the 

 otic capsule instead of backward over the squama of the occipital, as in the adult. 

 At the bottom of the transverse sulcus the exoccipital joins the mastoid portion of 

 the temporal. Along the upper part of this junction young cartilage is present, but 

 towards the jugular foramen the two are joined by precartilage and at the jugular 

 foramen by dense mesenchyme or blastema, which in older embryos changes into 

 cartilage. There is no line of demarcation between the exoccipital and the squama. 

 The latter continues upward from the exoccipital as a broad, curved plate of cartilage. 



Above the region of the exoccipital the occipital squama is continuous with 

 the mastoid portion of the temporal. There is no line of demarcation between the 

 squama and mastoid, and therefore much confusion exists in the Literature regard- 

 ing this region. This has come about through the use of the term parietal plate 

 for the plate of cartilage lying above the outer edge of the otic capsule and partly 

 continuous with it. We naturally associate the word parietal with the membrane 

 bone of the same name, and the assumption has been that the parietal plate dis- 

 appeared at a later stage and was replaced by the membrane bone. Its fate, how- 

 ever, has never been carefully traced, and it is probable that this so-called parietal 

 'plate, which extends backward into the nuchal plate, is part of the mastoid, for in 

 the adult the mastoid is continued upward, around the posterior and even the dorsal 

 edge of the petrous bone. 



The foramen magnum is incomplete dorsally except for a thin membrane of 

 connective tissue, the dorsal membrane. In later stages (as shown by Levi, 

 Hertwig, and Macklin) the foramen is completed by a bridge of cartilage formed 

 by the junction of the occipital squamae, which gradually extends around the 

 brain. This cartilaginous bridge or band, usually called the tectum posterius 

 or tectum synoticum, in older stages extends around the brain from the lateral 

 occipital and mastoid plate of one side to the lateral occipital and mastoid plate 



