No. 2.] I'ERTEBRATE CEPHALOGENESIS. 243 



characters have been impressed upon thcvi by other organs of seg- 

 mental nature. Not ojtly does ontology force this conclusion, hit 

 the historical dcveloptnent of our anatomical knozvledge alike com- 

 pels its admission. 



It is taken for granted that the head region has been formed 

 by a gradual process of modification of the anterior end of some 

 ancestral type, whose body, with the exception of the first and 

 last segment, was composed of nearly, if not quite, homodyna- 

 mous segments. The problems are to determine what was the 

 value of these primitive segments, and how they have been so 

 modified as to produce the head. Obviously the only way open 

 to the morphologist is to determine what existing vertebrates 

 show the steps of this process, and the extent to which the 

 primitive segmentation persists in the highly differentiated ver- 

 tebrates of this age. 



Few if any topics of vertebrate anatomy have received the 

 attention of so many morphologists, or have been discussed 

 with such interest, as the nature and development of the head. 



It has long been recognized that within the vertebrate phylum 

 the cephalon undergoes a most wonderful degree of differen- 

 tiation, unequalled by any other portion of the body. As the 

 lowest step in this series we have Amphioxus with the head less 

 developed, both morphologically and physiologically, than many 

 annelids and arthropods, — with this reservation of course, that 

 in the Lancelet, as we say, the type is higher. From Amphioxus 

 to the next stage, as represented by the Cyclostomes, the transi- 

 tional forms are unknown, but the larval development of Petromy- 

 zon gives some interesting hints as to the manner in which 

 these transitional stages have been passed through. From the 

 Cyclostomes up to man, the series is practically unbroken, and 

 allows us to trace, with a great degree of accuracy, the course 

 of development, and to formulate some ideas as to the causes 

 which have been at work effecting the progress. 



Morphological conclusions, in the realm of neurology, have 

 undergone great modification within the last decade, and as 

 our knowledge increases we have found it necessary to widen 

 our views as to what is normal and essential for a neural seg- 

 ment. From a general survey of the field, it is clear that a 

 classification of the cephalic structures of modern vertebrates 

 must be made, separating those which are genetically connected 



