122 BIOLOGICAL LECTURES. 



the jointed condition is designated "metamerism of the head." 

 Under that title the question has recently received much atten- 

 tion. It is not a new question, having been started at the 

 beginning of this century, but there has been a revived interest 

 in it on account of new discoveries. It was the most promi- 

 nently discussed question before the Anatomical Society of 

 Germany, at their meeting in Vienna, in 1892. There were 

 important papers on the subject by some of the foremost 

 anatomists, — Froriep, Kupffer, Hatscheck, Rabl, Killian, — all 

 followed by discussion, and the question of metamerism of the 

 head received there a consideration worthy of its importance. 

 There have been new developments since that time, tending to 

 modify some of the conclusions reached in that learned body, 

 and the subject is, on account of its freshness, a particularly 

 good one to present here. 



The presence in the head of such segmental structures as 

 cranial nerves, branchial clefts, of the adult and embryonic 

 stages, the so-called head cavities and neural segments, have 

 been sufficient evidence to support the general proposition that 

 the vertebrate head is segmented in its unmodified condition. 

 But there has been no agreement as to the number, nature, 

 or transformations of these segments, nor as to their anterior 

 limit. In fact, the fore-brain has generally been regarded as 

 not included in the segmented area. 



Segmental folds have been observed in the hind-brain of 

 embryos of different animals since 1828, so there has been no 

 question as to the segmented condition of the posterior part of 

 the brain, but, so far as the evidence (except the very latest) 

 goes, the segments seem to vanish in the region of the fore- 

 brain, and the general interpretation has been that the fore- 

 brain is non-metameric. The two anterior pairs of nerves com- 

 ing from the fore-brain — olfactory and optic — have also until 

 very recently been placed, by common consent, in a different 

 category from the other cranial nerves. Moreover, the head has 

 been considered, by some of the most careful students of our 

 time, to be derived from an unsegmented ancestral rudiment 

 found in an enigmatical larval form of the annelids. All this 

 gave rise to the assumption that the brain of invertebrates and 



