CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 319 



The occipital squama or nuchal plate springs from the upper border of the 

 lateral part of the jugular process. Near its origin it is narrow in the sagittal plane 

 and wide in the frontal plane. It is continuous with the basal part of the lamina 

 by a curved plate, the alar lamina (figs. 1 and 16), and rapidly widens into a broad, 

 thin plate, which continues upward to a thin edge that runs into the dorsal mem- 

 brane. Both the jugular process and the squama extend into the mastoid cartilage. 

 At the jugular foramen these two cartilages are separated by blastema and precar- 

 tilage; above this they are joined by young cartilage, which gradually changes into 

 cartilage indistinguishable from that of the squama or mastoid. The lateral surface 

 of the squama is covered by condensed mesenchyme into which the edges of the 

 cartilage merge. It is into this mesenchyme that the occipital muscles namely, 

 the trapezius, splenius capitis, semispinalis, rectus capitis posticus major and minor, 

 and the obliquus capitis superior, appear to be inserted (figs. 14 and 15). 



THE SPHENOID CARTILAGE. 



The sphenoid cartilage consists of a body with two lateral wings or processes 

 on each side, the temporal and orbital wings (figs. 1, 2, 3, and 6). The cartilage of 

 the body is continuous with the body of the occipital, and in front narrows and 

 thickens as it passes into the nasal septum. The cartilaginous center from which 

 the body develops is described as being caudal to the hypophysial canal. The 

 chondrification spreads caudally to meet the basioccipital, upward into the dorsum 

 sellae, and forward around the hypophysial canal where the cartilage fuses around 

 the canal and extends forward to form the anterior part of the sphenoid and the 

 nasal septum. Fawcett, after an examination of a 21 and a 19 mm. embryo, con- 

 cluded that the dorsum sellse was an independent formation, since he found in each 

 embryo a separate cartilaginous bar above the region of the body and separated 

 from it by mesenchyme. Twenty embryos of the Carnegie Collection, ranging in 

 length from 15.5 mm. to 24 mm., were examined, and such a cartilaginous bar was 

 found in but one (No. 229), 19 mm. long. Twelve of the embryos were between 

 18 and 23 mm. Great variation in the form of the dorsum sellae was observed, but 

 the rather rare presence of a separate center of chondrification would indicate that 

 as a general rule the dorsum sella? develops from the basisphenoid, as described by 

 Levi. In front of the dorsum sellse the body is biconcave and perforated near the 

 center by the hypophysial canal. There is practically no indication in front of the 

 sella turcica of the tuberculum sellae. The anterior part of the body is very incom- 

 pletely developed, and not until the anterior tip of the orbital wing has grown 

 medialward to meet its fellow of the opposite side and fuses with the body is the 

 anterior border of the chiasmatic groove apparent (fig. 3). 



The temporal wing consists of two parts : the alar process, which springs from 

 the body just in front of the level of the dorsum sellse and projects laterally and 

 slightly caudally toward the apex of the cochlear part of the otic capsule, and the 

 lateral part, attached to the under surface of the alar process and projecting later- 

 ally. Each part is supposed to have an independent center of chondrification. In 

 this embryo the alar process is joined to the basisphenoid by young cartilage and 



