416 CHARLES CLIFFORD MACKLIN 



siderably older than mine. According to Mall ('06) the centers 

 for the interparietal (one on each side) appear on the 57th day 

 and unite on the 58th day. 



Situated some distance above the otic capsule, and forming a 

 part of the side wall of the cranial cavity, is the network of bone 

 which is the anlage of the future parietal (fig. 4). It is roughly 

 of diamond shape, with the long axis directed cranio-caudally, 

 and medial to its lower border is the disappearing parietal plate, 

 the two layers, cartilage and bone, being separated by a slight 

 interval filled with connective tissue. There is a considerable 

 area separating this bone from the frontal, and it is also widely 

 separated from the interparietal, there being a very large area at 

 the back of the cranium, behind the parietals and above the 

 tectum posterius, uncovered by bone or cartilage. 



The parietal bone is not so dense as the frontal, showing that 

 it is probably later in appearing. Its structure presents no 

 definite center, it being a fine reticulum throughout, though, for 

 technical reasons, it has been represented in the illustrations as 

 a solid plate. 



The frontal bone (fig. 4) is similar in structure to the parietal, 

 and encloses the lateral and ventral parts of the anterior cra- 

 nial fossa. In it two main parts may be distinguished, a ver- 

 tical portion, convex antero-laterally, and a horizontal portion, 

 which is slightly arched upwards for the accommodation of the 

 structures of the orbit ; these being the anlagen of the correspond- 

 ing parts of the adult bone. Joining these portions is a well- 

 marked, rounded ridge, the representative of the future supraor- 

 bital ridge, and here the bone is most densely deposited, especially 

 in the central part of its extent, indicating, probably, that in this 

 position the first masses of osseous tissue were laid down, and 

 not in the region where the frontal eminences will later appear, 

 as is stated in the textbooks. The entire margin of the bone, as 

 well as the greater area of the posterior half, is but a tessellated 

 plate composed of intercrossing osseous spicules. The anterior 

 extremity is rounded, and shows no definite resemblance to the 

 mature condition; it is separated by a considerable interval 



