CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 321 



the latter are more or less apparent by the configuration of the cartilage. The 

 position of the superior semicircular duct is quite clearly indicated; it lies within 

 the prominent, rounded semicircular mass constituting the antero-dorsal portion 

 of the canalicular half of the capsule (figs. 1, 2, 3, 5 and 15). The deep fossa sub- 

 arcuata lies at the center of its curve. The lateral semicircular canal produces a 

 distinct bulge on the lower part of the lateral surface of the capsule, while the 

 position of the posterior duct is indicated by the configuration of the posterior 

 portion of the canalicular part. The cranial surface shows a well marked sulcus 

 for the endolymphatic duct which leads into the opening for the aqueductus 

 vestibuli. The upper edge of the cartilage, where it joins the mastoid, is deeply 

 grooved for the transverse sinus. The lower edge of this posterior portion projects 

 ventrally in the form of a ridge, the crista parotica, to the caudal end of which is 

 attached the styloid process. In front of the crista there is a slight depression, the 

 fossa incudis, in which is lodged the short crus of the incus. The facial nerve lies 

 in a groove just medial to the crista and styloid process (figs. 6, 12, and 13). 



MASTOID CARTILAGE. 



The mastoid cartilage is continuous with the caudal and upper border of the 

 canalicular part. We have already described its fusion with the exoccipital and 

 squama of the occipital. Its inner surface is hollowed out for the great transverse 

 sinus, and in the upper part of this sulcus is the large mastoid foramen. A dis- 

 tinct mass of blastema or precartilage projects ventrally from the inferior edge of 

 the mastoid just back of the root of the styloid process. Since to this mass are 

 attached the digastric, splenius capitis, longissimus capitis, and sternomastoid 

 muscles, it probably represents the mastoid process. In addition to the above- 

 mentioned muscles the stapedius also appears to arise from the inner side of the 

 mass. At this stage the stapedius is relatively large and entirely extracapsular. 

 Two interesting rudimentary muscles, not found in the adult, are likewise attached 

 to this mastoid process. One, short and thick, arises from the occipital transverse 

 process; the other, longer and more slender, extends from the transverse process of 

 the atlas (figs. 14 and 15). I have named these muscles the occipito-mastoid and the 

 atlanto-mastoid. The occipito-mastoid is serially related to the intertransversarii. 



This mastoid blastema, or mastoid-process blastema, presents interesting 

 variations. In embryo No. 382 (Carnegie Collection), 20 mm. in length, there is 

 a small nodule of cartilage in this region the mastoid-process cartilage. This is 

 attached to the occipital transverse process by a short ligament. Both the digas- 

 tric and stapedius muscles are attached to it, but no trace of the occipito-mastoid 

 or atlanto-mastoid muscles were found. In embryo No. 431 (Carnegie Collec- 

 tion), 19 mm. in length, a similar nodule of young cartilage was found attached 

 in the same manner to the transverse process of the occipital by a ligament. The 

 stapedius muscle is attached to its inner surface and the digastric to its lower 

 surface. In both embryos the mastoid-process cartilage is closely associated with 

 the mastoid blastema and the ligament or band of condensed mesenchyme, connect- 

 ing it with the jugular process, occupies the same position as the occipito-mastoid 

 muscle found in embryo No. 460. The attachment of the stapedius and digastric 



