38 MARINE BIOLOGICAL LABORATORY. 



vertebrate characters is in the lower rather than the 

 higher forms, and in the embryos rather than in the 

 adults. The original expounders of the vertebral theory 

 pursued just the opposite course, and were thus deceived- 

 by superficial analogies. 



This theory, wide of the mark as it was in its origi- 

 nal form, contains a germ of truth, which embryology 

 has brought to light. Its fundamental idea, that the 

 head, in whole or in part, is composed of segments 

 morphologically equivalent to those of the body, may 

 now be said to be an established fact. The problem 

 as it now stands is this, — How far is the segmenta- 

 tion of the body carried forward into the head ? in 

 other words, how many segments are represented in the 

 head ? Let us look a little further into the nature of 

 the problem, and the methods and means of approaching 

 its solution. 



The subject is a difficult one to make intelligible to 

 those who are not familiar with the main features of 

 development ; but it is so full of instruction, that it can- 

 not fail to yield some points of interest even to a most 

 superficial examination. Possibly the idea of segmen- 

 tation of the head, or, to use the technical expression, 

 the metamerism of the head, may appear to some of 

 you quite devoid of general interest or importance. The 

 principal charm of the subject of course lies in its envi- 

 ronment, if I may use such a term, to express its general 

 relations or bearings. In attempting to decipher the 

 metamerism of the vertebrate head, we are really en- 

 gaged in tracing the history of the origin of the great 

 vertebrate stock or phylum. It is simply a question 

 of the genesis or the pJiylogeiiy of the vertebrate type. 



