AQUATIC MAMMALS 



further elongated, more teeth may be acquired at the end of the series 

 to fill the space provided. But we do not know how or why. 



Tooth buds are present in fetuses of the Mysticeti or whalebone 

 whales, but these are absorbed when the baleen starts to develop, or 

 even before. Flower (1893) considered that the baleen plates devel- 

 oped gradually over their entire present area from the oral ridges of 

 the roof of the mouth, which are present to some extent in most mam- 

 mals and are highly developed in ungulates. The baleen is analogous 

 to the ridges but Flower was mistaken in his premise and they are not 

 really homologous. I have examined a fresh eleven foot fetus of 

 Balaeuoptera in which the start of the baleen growth was to be seen 

 to good advantage. Slightly sunken within a groove which corre- 

 sponded in position exactly with a maxillary dental arch was a soft, 

 whitish tissue abruptly differentiated from the normal oral epithelium. 

 This was about 3 centimeters in width and continuous save for a brief 

 interruption anteriorly at the midline of the rostrum. It was plain 

 that as this strip grew to form the young baleen there would be an 

 accompanying widening in a medial direction so as to cover most of 

 the roof of the mouth, which at the stage examined was formed of 

 normal mucous membrane. So Tullberg (1883) was correct in prin- 

 ciple in stating that baleen develops from a growth of papillae along 

 the outer margin of the upper jaw. 



Of extreme interest in the above connection is a condition in Phoco- 

 enoides dalli to which Miller (1929) has recently called attention. In 

 a preserved section of the upper jaw of the specimen which he had 

 the tooth tips appeared as being sunk in small pits and were below 

 the gum surface so as to be nonfunctional. In compensation the gum 

 along the dental arch and immediately adjoining on either side had 

 developed rows of cornified papillae which had obviously taken over 

 the grasping function of the decadent teeth. Histologically the struc- 

 ture of these papillae certainly seems to be homologous with baleen. 

 So we appear to have an illustration in an odontocete of just how 

 and why the baleen first began to appear in the mysticete ancestry. 



Wlialebone whales may have nearly 400 plates of baleen on either 

 side. Flower and Lydekker (1891) stated that whalebone consists of 

 modified papillae of the mucous membrane, with an excessive cornified 

 epithelial development, there being at the base and between the blades 

 an intermediate substance consisting of a softer epithelium. The latter 

 is white and of a cheesey consistency, and projects into the base of each 

 blade. When an animal is examined in the flesh the baleen appears 



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