35o DISCOVERY REPORTS 



the vomer between them. The foetal brain case is of great size and domed, while the 

 lateral projections of the frontals and squamosals are very small in comparison with 

 those of the adult, where the squamosals form two large postero-lateral downwardly 

 curved wings. 



The base line of the foetal skull is horizontal and the whole plane of the basis cranii 

 broad and flat, while in the adult it curves strongly downwards from the great wing-like 

 projections of the maxillae. The foetal maxillae are of comparatively small size, while 

 the palatines and pterygoids are large, the latter projecting backwards on either side of 

 the posterior nares to form two rounded bosses — the hamular processes (Fig. 8). Schulte 

 (1916, pp. 476-7; pi. 54, fig. 2; pi. 55, fig. 2) distinguished two parts of the ptery- 

 goid bone of the foetal Sei Whale, an internal and an external pterygoid respectively, 

 separated by a suture. Ridewood (1922, pp. 263-4), however, did not confirm the 

 presence of this suture in the skull of the Sei Whale foetus examined by him, nor in 

 the skulls of foetal Megaptera nodosa. This author believed that the pterygoid bones 

 figured by Schulte are in reality parts of the palatine. 



While the predominating features of the adult basis cranii are the laterally projecting 

 squamosals and the large wing-like maxillae, the predominating feature in the foetus is 

 the enormous tympanic bulla with its tube-like meatus. It is joined to the mandible by 

 a slender strip of cartilage — Meckel's cartilage — enclosed in a fibrous sheath. In the 

 adult the bulla is a relatively very small fist-shaped bone concealed on the inner side of 

 the squamosal wing. It forms a quite insignificant feature of the occipital region. 



In the foetus the basicranial rete lies on the ventral surface of the squamosal bone, in 

 a cavity between the enormous round dome of the tympanic bulla and the articulation 

 of the mandible. It extends over the external pterygoid plate and the alae temporales 

 into the orbit. It thus occupies a large crescent-shaped space in the foetus. In the 

 adult, however, the dimensions of this space are much reduced owing to the shrinkage 

 of the bulla and the pterygoid and the development of the fibrous articulation of the 

 mandible which occupies most of the squamosal wing. So far as could be made out the 

 greatest extension of the adult rete is vertical, so that it forms a fatty vascular mass, 

 similar in appearance to the thoracic rete, extending upwards into the fissure between 

 the squamosal and the basioccipital bones, where lies the small fist-like tympanic. It 

 appears also to extend on to the pterygoid in the adult, but this part is concealed by the 

 broad internal pterygoid muscle. 



RELATION TO MUSCLES 



The cavity occupied by the rete in the foetus thus extends from the orbit to the 

 extreme posterior limit of the tympanic bulla (Fig. 9). It underlies the tympanic 

 ventrally, and is laterally bounded by the masseter muscle and the attachment of the 

 pterygoid muscles to the mandible. The broad sheet-like internal pterygoid muscle 

 forms the floor of the space, and the mesial boundary of it is made up of the palato- 

 glossus muscle and the middle constrictor of the naso-pharynx. The palato-glossus has 



