SKULL OF A HUMAN FETUS OF 40 MM. 369 



marks the zone of union with the squama. Several small blood- 

 vessels are found in the ventral portion of the occipitoparietal 

 groove, but in its dorsal part there is but a single small vessel. 

 Just above the groove, and running parallel with it, is a low 

 rounded ridge. The dorsal, scimitar-like extremity of the 

 parietal plate is shown projecting freely dorso-medially. The 

 outer surface is convex and is biit indistinctly marked off from 

 the underlying squama. 



In the membrane forming the posterior and superior part of 

 the cranium, considerably above the tectum posterius, are to be 

 seen two small cartilages lying side by side, the cartilagines 

 cranii posteriores. The cartilage on the right side, though small, 

 is relatively very large when compared with its partner of the 

 left, which is insignificant. 



These cartilages appear to represent the unpaired mass de- 

 scribed by Bolk as lying above the tectum synoticum, which 

 subsequently disappears. Possibly in my embryo they are under- 

 going reduction. Mead describes a somewhat similar small free 

 nodule in Sus, but this is single and, although in the midline, 

 it lies immediately above the tectum. He calls it the processus 

 ascendens of the tectum posterius, and thinks it may possibly be 

 the homologue of the processus ascendens of the tectum posterius 

 of reptiles. 



