44 MARINE BIOLOGICAL LABORATORY. 



I must now invite you to the very borders of the 

 beaten ground of investigation ; and I hope you will 

 have the fortitude to follow me even to the brink of a 

 precipice or two, should it be necessary, in order to get 

 a view of the steep ascent which now challenges further 

 advance in this direction. I have endeavored to give 

 you the salient points in the historical development of 

 the subject, and it remains for me to define the position 

 now occupied, and so far as possible, by way of anticipa- 

 tion, the path which investigation is destined to take in 

 the immediate future. 



In the anterior region of the head, into which the 

 cordal axis or "primitive backbone," as Lankester has 

 called it, does not extend, there are two sensory nerves, 

 the olfactory and the optic, which investigation has thus 

 far failed to reduce to the type of the spinal nerves. No 

 corresponding motor nerves exist ; and no decisive evi- 

 dence of metamerism has yet been discovered in their 

 development, or adult condition. Foremost authorities 

 in anatomy and embryology, like Gegenbaur, Balfour, 

 and Kolliker, have declared that here a dividing line 

 must be drawn, separating the head into two distinct 

 regions, one of which bears with the trunk the common 

 stamp of metamerism, while the other is built upon a 

 plan of its own. It is here that Balfour, looking back 

 into the remote ancestral history of the vertebrates for 

 clews, recognized what appeared to him a primitive boun- 

 dary line, corresponding to what now divides the head 

 and trunk in many invertebrate forms. According to 

 this view, the fore-brain would represent the whole of 

 the ancestral brain, while the mid-brain and hind-brain 

 would represent a number of segments belonging origi- 



