198 Journal of Comparative Neurology and Psychology. 



Indeed it may be said that our attitude toward the facts which 

 determine the location of the anterior end of the head largely 

 determines our attitude toward the whole head problem. 



The questions of the anterior end of the brain and of the 

 head are inseparably connected. The anterior end of the brain 

 may mean two things according as we consider the axis of the 

 brain cavity or the mid-line of the medullary plate. If the 

 neuropore is the open end of a tube which is equally long dor- 

 sally and ventrally, then the anterior end of the axis of the 

 cavity would lie in the same dorso-ventral plane as the anterior 

 end of the mid-line of the medullary plate, and that portion 

 of the tube which forms the border of the neuropore would 

 form a ring every point of which falls equally near the anterior 

 end of the head. As such an assumption begs some of the 

 questions at issue it is inadmissible, and we must take the 

 safer course of considering the primitive brain as a flat plate 

 whose anterior end is marked by the point at which its mid- 

 line passes over into the general ectoderm. It is customary to 

 say that this point is at the lower border of the neuropore. 

 This definition of the anterior end of the brain would be satis- 

 factory if it were shown that the lower border of the neuropore 

 occupies the same position in all vertebrates with reference to 

 other structures of the head and with reference to structures 

 within the brain itself. It is probable that it does not. It is 

 necessary, therefore, to take a wider view and determine the 

 front end of the head as a whole. 



The anterior end of the head is marked by thai point at 

 which the brain plate meets the general ectoderm at the same time 

 that it comes into contact with the anterior end of the entodertn in 

 those vertebrates in which the praeoral entoderm is most completely 

 prese^i'ed. This definition does not indicate in advance any 

 particular point that must be accepted as the anterior end of 

 the head. There is in every vertebrate embryo a common 

 point at which brain, ectoderm and entoderm meet. It is a 

 most reasonable assumption that for the given animal such a 

 point marks the anterior end of the head. Following this 

 mode of reasoning Koltzoff {j6) locates the anterior end of 



