CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 313 



blastema of the mastoid process to which are attached the digastric and stapedius 

 muscles. The serial relationship shown in the embryo is quite different from that 

 of the adult, where the mastoid process which gives attachment to the digastric 

 is some little distance lateral to the jugular process, and the styloid process is im- 

 mediately in front of the latter. Just in front of the jugular foramen is the opening 

 for the aquaeductus cochleae. 



The sphenoid shows the basisphenoid with the opening of the hypophysial 

 canal near its center and the two wings on each side. Part of the orbital wing is 

 obscured by the lateral part of the nasal capsule, as it would be in the adult from 

 this point of view. 



The line of fusion of the nasal capsule with the mesethmoid is shown, and the 

 anterior nares directly face the observer as they do in the adult. The junction 

 of the nasal capsules with the mesethmoid is effected along the middle third of the 

 anterior edge of the mesethmoid by precartilage. The cut edge of this junction 

 is shown in figure 5. 



LATERAL ASPECT. 



The cartilaginous skull covers but a small area of the lateral surface of the 

 brain, namely, part of the medulla, part of the cerebellum, and a small area in the 

 region of the optic nerve (figs. 7 and 8). The cartilaginous skull, even in its 

 completed form, is a very inadequate protector for the brain, never covering more 

 than a small fraction of its surface. The condensed mesenchyme or blastema 

 covers a much larger surface, but even the cartilage and blastema together form 

 at this stage a very incomplete brain capsule (fig. 9). The capsule is completed 

 by the thin dorsal membrane. The inclosing of the central nervous system by the 

 gradual spreading of the blastema and cartilage, which invade and replace the dorsal 

 membrane, is similar to the well-known development of the thoracic and abdominal 

 walls and the disappearance of the ventral membrane. 



The blastema covers almost all of the lateral surface of the cartilaginous skull. 

 A small part of the occipital cartilage, including the transverse process, part of the 

 squama and occipital neural arch, part of the orbital wing of the sphenoid, and 

 part of the lateral surface of the nasal capsule, are uncovered (figs. 9 and 15). 

 Into the blastema covering the squamal cartilage, rather than into the cartilage 

 itself, are inserted the various occipital muscles (figs. 14 and 15). The blastema 

 covering the squama and the lateral surface of the otic capsule probably fuses 

 later with the perichondrium, but at this stage it seems to be continuous with the 

 rest of the blastemal wall which later gives rise to membrane bones. It is in the 

 sphenoidal and frontal regions that the blastema greatly predominates over the 

 cartilage. All of the lateral wall of the middle cranial fossa consists of blastema 

 and the greater part of the floor (as well as all of the lateral wall of the anterior fossa) 

 is likewise formed by blastema. The orbital walls are mostly of blastema; cartilage 

 of the orbital wing of the sphenoid takes part in the formation of the apical region 

 of the orbit about the optic foramen, and a portion of the medial wall of the orbit 

 is formed by part of the lateral wall of the nasal capsule. Connected with the 



