60 



SKULL OF A 43-MM. HUMAN FETUS. 



DESCRIPTION. 



THE SKULL AS A WHOLE. 



The skull of No. 886, although considerably 

 younger than that of la, resembles it quite closely, 

 so that I have not been put to the task of writing 

 such a detailed account as would have been 

 necessary had my former article not been avail- 

 able. I have endeavored to avoid a repetition of 

 my former description, and to make this article 

 largely a comparison of No. 886 with 7a. 



Figure 1 presents the most favorable view from 

 above. The basal plate is not exactly horizontal, 

 the cranial end being a little closer to the eye of the 

 observer than the caudal. The nasal capsules are 

 seen from an oblique direction, and hence a true 

 concept of their length is not obtained, as will be 

 realized when the parts are regarded from the 

 side. 



By imagining the frontal and parietal bones 

 as having been inserted on the left side, as well 

 as on the right, and by filling the gaps between the 

 elements of the outer border, it is seen that the 

 contour of the cranial cavity, from this point of 

 view, is oval, and a little wider anteriorly, in the 

 region of the frontal bones, than posteriorly, at 

 the widest part of the parietal plates and otic 

 capsules. Posteriorly this contour is made up of 

 the cartilaginous walls of the future posterior 

 cranial fossa; anteriorly, however, it is composed 

 of membrane and membrane bone. The chon- 

 drocranium is divided into two unequal portions 

 in the region of the body of the sphenoid by the 

 superior orbital fissure — an extension of the spheno- 

 parietal fissure. Projecting into this fissure from 

 the body of the sphenoid is the short temporal 

 wing. 



If this figure be compared with figure 1 of la we 

 note obvious signs of advancement in the latter, 

 perhaps the most marked being the more de- 

 veloped state of the anterior end of the skull. 

 Lewis (1920) has commented upon the relatively 

 more rapid growth which must take place in the 

 prechordal, as contrasted with the chordal, portion 

 of the skull, following the stage of 21 mm. which 

 he studied, and others have expressed themselves 

 similarly. From a comparison with No. 460, on 

 the one hand, and with la on the other, it is very 

 apparent that in No. 886 development of the 

 anterior end of the skull is going on more rapidly 

 than that of the posterior end. 



Certain features of la, such as the sharp bending 

 of the otic capsule, suggest that the specimen from 

 which the model was made was laterally com- 

 pressed and somewhat shrunken. 



In figure 2 we have the most favorable aspect of 

 the skull base. In it the basal plate is almost 

 horizontal, the caudal end being a little closer to 

 the eye of the observer. The view is thus not 

 directly antipodal to figure 1, and hence the outer 

 contours of the entire skull, and of the chondro- 

 cranium, are slightly different. The ethmoidal 

 region is viewed from the anterior end, making 

 impossible an adequate appreciation of its length 

 and that of the fissura basalis, which separates the 

 ectethmoid from the mesethmoid. The lower 

 jaw has been entirely cut away. 



A frank view of the face is seen in figure 3, 

 which also includes partial views of the larynx and 

 cervical vertebra?. The gaping mouth and orbits 

 are conspicuous features. 



The skull is so placed in figure 4 that the eye 

 looks squarely into the foramen occipitale mag- 

 num. The vertebral column is tilted a little so 

 that its lower end is slightly nearer to the eye than 

 the upper. The skull is not absolutely sym- 

 metrical and there is a slight deflection of the 

 axis of the basal plate to the left, as the figure 

 shows. 



Side views of the skull are afforded by figures 5 

 and 6, the former including the membrane bones. 

 The depth of the posterior cranial fossa is ap- 

 preciated by looking at the dorsal portion of the 

 chondrocranium. The hyoid, thyroid, cricoid, and 

 four upper tracheal rings are seen. The contours 

 of the external form, brain, frontal and parietal 

 bones, and chondrocranium are seen from the 

 right side in figure 7 in their normal relationship. 

 This figure was made by profile reconstruction. 

 The other figures show various details of the 

 chondrocranium, and will be referred to in the 

 description. 



In general form the skull of No. 886 resembles 

 closely the 28 mm. stage of Levi (1900), which 

 seems to be a little farther advanced than the 30 

 mm. stage of Jacoby (1895). 



CENTRAL STEM. 



The central stem of the chondrocranium is seen 

 from the side in figure 10, with the cut surfaces of 

 its adnexa indicated. Its chordal and prechordal 

 limbs meet in the region of the hypophyseal fossa, 

 where the stem appears to have been twisted 

 through an angle of 90°, as well as strongly bent. 

 The chordal limb represents the basal plate of the 

 skull, while the prechordal limb contains the body 

 of the sphenoid, with the interorbital and nasal 



