SKULL OF A 43-MM. HUMAN FETUS. 



81 



tinuous medially with the medial mass, near the 

 anterior end. The lateral edge is situated a little 

 higher than the medial, making the lower surface 

 look somewhat outward as well as downward. 



The medial mass is a vertical plate of very young 

 cartilage, edged with precartilage, which shows 

 but little of the differentiation that characterized 

 its condition in la. In shape, instead of being 

 roughly quadrilateral, as in la, it is like a right- 

 angled triangle, the projecting right angle being 

 situated caudoventrally. The horizontal base is 

 notched. The posterior side, longer than the 

 base, is vertical in position and is bowed outward 

 a little about its middle. It terminates below in a 

 very slight enlargement. Its outlines are indefi- 

 nite, merging into the surrounding mesenchyme, 

 and it is of precartilage. The vomer is some dis- 

 tance behind it and is situated closer to the mid- 

 line. The hypotenuse runs practically parallel 

 with the lower edge of the septum and is but lit- 

 tle above this, so that the portion of the septum 

 lying between these plates is very small. The 

 anterior angle is continuous with the septum 

 (fig. 10) and with the fundament of the ventro- 

 lateral process. The latter connection was not 

 present in la. This end probably represents the 

 cranioventral process of la. A connection with 

 the lateral mass, just behind this region, seems to 

 represent the position of the ventral process of la. 

 The surfaces of the plates look almost directly out- 

 ward and inward. They are separated by an 

 interval but little wider than the lower edge of the 

 septum and lie practically parallel with one 

 another. 



These cartilages, being of such young material, 

 are evidently developing rapidly and would 

 doubtless soon have become differentiated as in la. 

 They are somewhat difficult to outline, for they lie 

 in a mesenchyme of which the parts, as modeled, 

 are often but the most condensed portions. No 

 connection could be made out between the lateral 

 mass and the paraseptal process of the ectethmoid, 

 which project toward one another and which are 

 linked by a zone of loose tissue in la. 



ECTETHMOID. 



The ectethmoid is a thin crumpled plate of car- 

 tilage which presents on the outer surface a num- 

 ber of eminences and depressions, which appear 

 on the inner surface in reverse. In addition, the 

 inner surface shows the developing conchse. 

 In the ectethmoid there are represented a roof, a 

 lateral wall, and a floor, though the first and last 

 are not extensive. 



The roof occupies little more than half of the 

 entire length of the ectethmoid and is bounded 



laterally by the attachment of the spheno- 

 ethmoidal cartilage. The border is completed 

 anteriorly by the upper edge of the inturned 

 ectethmoid which meets the crista galli; the 

 boundary of the posterior part of the roof is 

 indefinite, being marked by a low medially curving 

 ridge, which ends somewhat behind the para- 

 ethmoidal process in the region of the limbus 

 sphenoidalis. 



Most of the roof is incomplete, showing ante- 

 riorly the wide fenestra cribrosa (fig. 1). Its 

 margin presents no cribro-ethmoidal process, as 

 in la. Posteriorly the roof is joined to the nasal 

 septum, and is of young cartilage. Some four or 

 five small holes appear in this evidently newly 

 formed part. Behind this area the roof can 

 hardly be said to exist, being represented only by 

 the sharply rounded upper edge of the antorbital 

 plane. The posterior part of it underlies the 

 preoptic root of the lesser wing. 



The side wall is irregular in formation and an- 

 teriorly turns inward to meet the nasal septum. 

 Underlying the lesser wing and spheno-ethmoidal 

 cartilage the upper border flares outward and thus 

 gives rise to a shallow and wide groove below. 



The posterior or orbital portion is flat and shows 

 near its upper extremity the small para-ethmoidal 

 process of young cartilage. This has a blunt, 

 free posterior end. In la there was here a free 

 nodule of cartilage, which lay very close to the 

 wall. 



The upper border is connected anteriorly with 

 the nasal septum through the roof; posterior to 

 this it turns sharply downward to terminate in a 

 free and very thin edge which, however, is con- 

 tinued downward beside the nasal septum as a 

 membrane. The wedge-shaped presphenoid lies 

 between these downturned parts of the border, 

 and so they are widely separated behind. They 

 are separated from the presphenoid by the very 

 narrow cupulo-septal fissure. 



The posterior end of the antorbital plane is 

 somewhat indented, but does not show such a 

 sharp dorsal palatine notch as did 7a. The upper 

 corner, though turned downward as a sharp 

 point of young cartilage, does not turn forward to 

 form a distinct posterior cupular process, as in 7a. 

 The ventrocaudal corner shows a backwardly pro- 

 jecting point of young cartilage. 



The lower part of the plate turns in sharply 

 to form the inferior concha. At the anterior end 

 of the angle thus formed is the posterior nasal 

 prominence, bearing the posterior maxillary proc- 

 ess, formed of young cartilage. 



The posterior maxillary process is considerably 

 smaller than in 7a, and between it and the pos- 



