620 JOHN D. KERNAN, JR. 



this inferior border of the external surface, a slender process of 

 cartilage juts forward which Macklin interprets as the anlage of 

 the tegmen tympani (fig. 2, 5). 



The base of the pyramid is ventral (fig. 3). It is roughly 

 triangular in shape, with the apex of the triangle covered by the 

 pars cochlearis. The external border is marked by the series of 

 processes already described, to wit: the mastoid process, the 

 crista parotica, the tegmen tympani. To the middle of this bor- 

 der is attached the short process of the incus. At the dorsal 

 extremity of the crista parotica, Reichert's cartilage is received 

 in a depression of the cartilage (fig. 5, 18). 



The parietal plate (fig. 2, Jf) is a thin sheet of cartilage which 

 surmounts the pars canalicularis and is continuous with it. 

 Ventrally this ends in a pointed process overhanging the capsule. 

 Dorsally it continues as a thin ribbon of cartilage, turns inwards 

 and meets the process from the opposite side. Thus is enclosed 

 the primitive foramen magnum. 



The ventral borders of these parietal plates are continuous 

 with the occipital wings, though the line of union is marked by a 

 groove on the inner surface, the occipito-parietal groove (fig. 

 1, 2). Thus they seem in this embryo to have a more intimate 

 union with the otic capsules than with the occipital wings, and 

 they may be considered as forming a true tectum synoticum. In 

 the 40 mm. stage Macklin found a more ventrally placed union of 

 the occipital squama, (the tectum posterius) . Above this union 

 he found the parietal plates to end with free dorsal edges. Levi 

 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. The conditions in Macklin's 40 mm. and this 

 20 mm. embryo would appear to bear out this statement of Levi. 



The significance of the mode of union of the two sides would 

 seem to be that the tectum synoticum is a more primitive 

 structure and is so formed early, and is absorbed as the tectum 

 posterium reaches its development. 



Jacoby was the first to point out that the parietal plates are 

 temporary structures which are early absorbed. Indeed, m the . 



