EMBRYOGENY 



215 



arise in the embryo not all at once ; they are gradually 

 constructed. A stage, therefore, which can be recog- 

 nized as such a beginning from its actual subsequent 

 fate, has absolutely nothing to do with a rudiment. 1 



Examples of actual retrogression are not rare. 

 In some whales (e.g. Ealcena mysticetus Greenland 

 Whale) there are found entirely buried in the flesh 

 some remains of the pelvis and the upper and under 

 thigh-bone, both imperfectly formed. Externally, of 



FIG. 47. BALJENA MYSTICETUS (Greenland Whale). (After Heffe.) 



hinder extremities there is nothing now perceptible 

 (Fig. 47). 



It is clear that these bones form no longer a function- 

 ally capable leg. We assume, therefore, in order in any 

 case to have a reason for their existence, that they are 

 rudiments (remains) of formerly normally constructed 

 extremities which have become superfluous owing to 

 the adaptation of the animal to life in the water and 

 which appear destined to disappear entirely. In other 

 species of whales there is no longer a trace. 



The Seals (Sea Lions) show also a rudimentary 



1 A renowned ' rudiment ' of this kind we hear of in the gill slits of 

 mammals and man. 



