NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 5 I 



makes its abandonment necessary, inasmuch as only a part of the 

 milk dentition, only four cheekteeth, were ever supposed to be inter- 

 calated in the permanent set (it was expressly said that an increase in 

 the number of cheekteeth by other means, by division or by the 

 formation of new elements, was necessary as an explanation when 

 the number exceeded ii in each jaw) ; and the milk dentition could 

 perhaps be made whole again after having given up some of its 

 contents. In any event it is impossible to trace in detail the destiny 

 of the tooth-germs through their erratic course during atrophy ; there 

 are many possibilities. 



In spite of all investigations during recent years into the develop- 

 ment of cetacean dentition both embryologically and phylogenetically, 

 no certain conception has been reached. Only this is certain, that the 

 cetacean dentition is derived from the typical carnivore's, that the 

 teeth are increased in number while their size is decreased and their 

 form made more simple, and that the tooth succession has essentially 

 ceased, although in embryos there can still be detected traces of 

 several sets of teeth, as in many other mammals, faint mementos of 

 their forefathers among the reptiles. But of how the changes have 

 in detail come about we can only partly guess. 



Abel is the author who has most recently reviewed the present 

 question. He believes that he can more nearly show how the high 

 number of teeth has arisen in the Cetacea ; in the whalebone whales 

 he thinks it came about in one way, and in the toothed whales, or at 

 least in the Physeterine series, in another. But his arguments are not 

 irrefutable. 



As regards the whalebone whales Abel starts from Kiikenthal's 

 investigations. As has long been known from observations by 

 Geoffroy Saint-Hillaire and especially by Eschricht, there is found 

 in embryos of all recent whalebone whales, hidden in each jaw, a long 

 row of small atrophied teeth with conical or knob-shaped crowns, 

 which are resorbed without ever erupting. Frequently some of these 

 small teeth are seen to be mutually united ; most often it is two that 

 come together but in rare instances as many, as four may unite. A 

 part of his observations on the embryonic teeth of Balmioptera 

 niuscitlus, the species which he has had especially good opportunity 

 to investigate, Kiikenthal summarizes in the following words : " Die 

 Zahl der Zahne im Oberkiefer des letzteren Embryos (that is, the 

 largest of those examined) ist 53; sie liegen sammtlich in gleich 

 weiten Abstanden von einander. Bei den kleineren Embryonen 

 betragt die Zahl der Oberkieferzahne, wenn wir die mit zwei resp. 



