120 BIOLOGICAL LECTURES. 



and complicated nature. A research once begun leads out in 

 all directions into the unknown, and a crop of new problems 

 springs up in the path of the original investigator. It is en- 

 couraging to biologists to know that every research serves 

 to enlarge the field of their activity ; the conquered territory 

 affords points of vantage from which the horizon is enlarged. 

 Thus the work of morphologists and physiologists on the 

 head, although not giving a complete solution to the problems 

 undertaken, has brought us larger views and more interesting 

 and suggestive lines of inquiry. 



Out of the great group of subjects connected with the 

 morphology of the head, I have chosen one that relates to the 

 primitive segmented condition. If we propound the question, 

 What was the rudimentary condition of the vertebrate head? 

 we shall find it may be partly answered in the light of modern 

 research as follows : It was originally composed of a series of 

 similar segments that were structurally like those that compose 

 the trunk. A mental picture of this rudimentary condition 

 may be formed by thinking of the broad cephalic plate (or 

 rudimentary head) of very early embryonic stages as divided 

 by transverse constrictions into ridges and furrows that pass 

 backward from the head into the trunk, and give the whole 

 embryo a jointed structure similar to that of an articulated 

 animal. The segments or folds do not cross the median plane, 

 and are therefore in pairs. From this simple condition the 

 complex head has arisen by differentiation and specialization. 

 It follows, of course, that the distinction between head-region 

 and trunk-region is one of degree of differentiation and not of 

 kind. 



The head is least modified in the youngest embryos, and if 

 we begin our observations with the earliest stages its trans- 

 formations may be traced by observing successively older 

 embryos. The modifications which the head has undergone 

 have been brought about gradually, and are so comprehensive 

 in their range that if we could know their complete history, 

 even in one animal, we should have a key to the leading ques- 

 tions of vertebrate descent. But there are so many causes 

 tending to modify the course of development, that we cannot 



