No. 3-] THE VERTEBRATE HEAD. 499 



The modifications which the head has undergone have been 

 brought about gradually, and are so comprehensive in their 

 range that if we could know their complete history, even in 

 one animal, we should have a key to the leading questions of 

 vertebrate phylogeny. But there are so many causes tending 

 to modify the course of development, that we cannot depend 

 on the steps of phylogenetic history being repeated in a 

 complete and orderly way in any one animal form. Often a 

 balance of probabilities must be struck to determine what is 

 ancestral and what is secondarily acquired. The chain of 

 evidence is indeed very incomplete, and must always be sup- 

 plemented by a certain amount of inference ; but what has 

 not been fully enough recognized in the practical study of 

 embryological development, is that the shortest intervals of 

 time may be very important in keeping the connection. 

 Coherency of the history must be preserved ; and the difficulty 

 of doing so is greatly increased by the fact that the new is 

 made to proceed out of the old, and, frequently, one organ 

 insidiously takes the place of an earlier formed one. Too 

 great stress cannot be laid on the desirability of having a more 

 complete series for study, and this is especially important in 

 cranial anatomy. The traditional method has been to study 

 one stage and then another " a little older," and to fill in the 

 intervening gap with inferences. This has proved to be 

 inadequate and misleading. It is now required that we shall 

 have stages close enough together to trace the history of the 

 transitory as well as the permanent organs. The practical 

 difficulties in obtaining a sufficiently complete series are very 

 great, and, in many cases, well-nigh impossible. Great effort 

 has been expended in getting the material for the present 

 research ; it is a kind of material in which the stages cannot 

 be controlled, and my series cannot be regarded in any sense 

 as a complete one. Nevertheless there is represented in it 

 several distinct stages that have not heretofore been described 

 by students of elasmobranch embryology. Beard, in his study 

 on the Transient Ganglion Cells and their nerves in Raja batis 

 says : " My series of the embryos of this form is what many 

 might judge to be complete, numbering as it does some 300 



