498 LOCK [Vol. XI. 



PAGE 



V. General Considerations 532 



1. JVew Aspect of the Problem of Metamerism. How to observe the 



segtnents in early stages 532 



2. Are they Artifacts ? 533 



(a) Different reagents used, (b) Coherent history in Acanthias. 

 (c) Presence in various kinds of embryos, (d) Observa- 

 tions on fresh material. 



3. Do they afford a Clew to the Meta7nerism of the Brain? 535 



4. They are formed Independently of Mesodermic Influence 536 



5. Presence of these Segments in Embryonic Rim and Primitive Groove 539 



6. Ntimber of Segments Represented in the Brain 539 



7. Relation of the Neurotneres to Sense-Organs and Cranial Nerves 541 



8. Head and Truttk 543 



9. Summary 544 



Part II. — The Sense-Organs. 



I. The Lateral Eyes 549 



II Accessory Optic Vesicles 556 



in. The Pineal Sense-Organs 561 



1. Growth of Knowledge regarding the Pineal Sense-Organs 561 



2. Remote Origin of the Pineal Outgrowth 565 



3. Cojnparison between Epiphysial Outgrowths in Petromyzon, Teleosts, 



and Lacertilia 571 



4. Double Nature of the Epiphysis 576 



5. Paraphysis 576 



IV. The Beginning of the Auditory Organ 577 



General Introduction. 



A GROWING interest has been manifested in the problems 

 of cranial morphology as their importance in comparative 

 anatomy has been more fully realized, and a number of strong 

 investigators have taken up this particular field of study. 

 Through their researches much has been accomplished ; the 

 views on cranial anatomy have gradually changed, as advance 

 after advance has been made, until morphologists have come to 

 look upon that wonderful complex — the head — not as a 

 structure sui generis, but as the most extensively modified part 

 of the animal, formed by differentiation and specialization 

 from parts that are structurally homologous with those that 

 follow in the trunk. According to this conception, the dis- 

 tinction between head region and trunk region is one of degree 

 of differentiation and not one of kind. 



