68 



SKULL OF A 43-MM. HUMAN FETUS. 



OTIC REGION. 



PARIETAL PLATE. 



The parietal plate (figs. 1, 5, 6, 14) is thin and 

 slightly concave en! ally. Its junction with the 

 squama occipitalis has been mentioned. An- 

 teriorly it is connected with the otic capsule by 

 the capsulo-parictal commissure which is inter- 

 rupted by the large and elongated capsulo- 

 parietal foramen. The plate, above the com- 

 missure, bends outward over a groove which is 

 bounded below by the otic capsule. 



The dorsalmost, irregularly rounded part of the 

 plate is the highest part of the chondrocranium 

 and is partially cut off from the main portion by 

 distinct notches, most closely approximated on the 

 right side. Its anterior edge does not overlap the 

 developing parietal bone, as in la, but is separated 

 from the posterior edge of this bone by a very 

 narrow interval (fig. 5). There are no small 

 cartilaginous remnants above it, as in la; indeed 

 this portion of the plate shows a relatively greater 

 development than in la, or (even more pro- 

 nounced) than in the Hertwig model, in both of 

 which it is evidently undergoing reduction. 



The anterior extremity of the main portion of 

 the plate is rounded and does not project far 

 beyond the capsulo-parietal commissure. Pos- 

 teriorly the plate curves inward, gradually nar- 

 rows, and ends in a point, separated from the 

 supraoccipital cartilage by the occipitoparietal 

 notch. The upper edge of the entire plate is 

 rather rough. 



DORSAL TECTA. 



The idea that there are two tecta represented 

 in the dorsal part of the occipital region has 

 recently been given attention by several authors. 

 Kernan (1916, p. 620) regards the tectum syno- 

 ticum, which joins the otic capsules through the 

 parietal plates, as a primitive structure which is 

 "formed early, and is absorbed as the tectum 

 posterium reaches its development." The tec- 

 tum posterius joins the occipital wings and thus 

 completes the foramen occipitale magnum. This 

 view Kernan arrives at from a comparison of 

 the findings in his 20-mm. human embryo with 

 those of my 40-mm. human embryo, together with 

 a consideration of the earlier evidence brought 

 forward by Levi. In his specimen Kernan de- 

 scribes the parietal plates (which are marked off 

 from the underlying occipital wings by the oc- 

 cipitoparietal grooves) as being joined behind 

 to form a true cartilaginous tectum synoticum. 

 In this way the primitive foramen magnum is 



completed, for the occipital squamae are not yet in 

 union. In 7a the parietal plates, although 

 stretching inward toward one another, do not 

 unite, while below them the occipital wings are 

 united in the tectum posterius. Levi, according 

 to Kernan, "showed that the dorsal union between 

 the two sides occurred cranially and advanced 

 caudally and ventrally, the more cranial union 

 between the parietal plates being absorbed as the 

 ventral union is formed." Thus Kernan remarks : 

 "The conditions in Macklin's 40-mm. and this 20- 

 mm. embryo would appear to bear out this state- 

 ment of Levi." This idea has been enunciated by 

 Rice (1920, p. 137). 



Fawcett (1918a, p. 227) writes: "In man there 

 are two tecta (Bolk, Fawcett) : one a very wide one, 

 the more posterior, therefore called the tectum 

 cranii posterius, from the middle of whose anterior 

 border a processus ascendens arises (Fawcett; in 

 pig, Mead) ; the other, the tectum cranii anterius, 

 is very slender and quite isolated, not reaching the 

 parietal plate on either side, nor being in any way 

 connected with the tectum cranii posterius. 

 Recently I have observed two tecta in the cat: one 

 certainly the ordinary tectum posterius, the other 

 small, median, and anterior to this, which may be 

 an isolated processus ascendens or may be looked 

 upon as a tectum anterius." In Weddcll's seal 

 Fawcett (19186) reports two cartilages, one on 

 either side of the midline, situated very far 

 forward, and belonging to the anterior tectum. 

 They showed slight signs of fusion with one 

 another. 



The condition in No. 886 does not add anything 

 to the information given by la, the parietal plates 

 ending as free points projecting into the dorsal 

 membrane. It is possible that the most anterior 

 cartilage of Bolk (1904) is to be looked upon as a 

 rudiment of the link which once joined the parietal 

 plates and, if this be so, then it is possible that the 

 posterior cranial cartilages of la belong to this 

 band. 



OTIC CAPSULE. 



In addition to the connections of the otic 

 capsule already noted, there is a small cartila- 

 ginous union with the processus alaris, one with 

 the incus, and one with the styloid process. 



As Lewis remarks for his 21-mm. specimen, the 

 capsule is placed in about the same position, with 

 regard to the basal plate, as is the petrous portion 

 of the temporal bone. Its shape, roughly that of 

 half a pear (with the large end situated dorso- 

 lateral^ and the cut surface toward the cranial 



