414 



method of body formation (under ordinary circumstances it is un- 

 doubtedly supplemented by growth by intussusception), but, there is 

 evidence to show that it occurs, in the usual train of events, and I 

 think that so far, Dr. Morgan's experiments may be accepted as 

 showing what is possible under materially changed conditions, and not, 

 as overthrowing the doctrine of growth by concrescence. 



In getting the problem of head metamerism clearly defined it 

 will be helpful to keep in view some general points. It is to be re- 

 membered, that the head region of vertebrated animals represents a 

 highly modified region. It is a complex that has been gradually 

 formed from a simpler condition. Also, that there is adequate evidence 

 to show, that the ancestral forms from which vertebrates have 

 descended, were divided into segments. If, therefore, we carry our 

 thought far enough backwards, we shall reach a simple condition in 

 which the head is imperfectly differentiated, and in which, therefore, 

 the head somites exhibit no marked distinction from those of the 

 body. This very condition repeats itself in the course of development 

 of the individual, and we have illustrations of it in figs. 2, 3, 4, 5 etc. 

 No one is likely to question but what the segmented condition I have 

 described, for Squalus, represents a survival, and I wish to call 

 attention again to its extremely early appearance in the development. 

 It is one of the earliest characteristics to arise, it antedates the 

 myotomes, the branchiae, cranial nerves and other segmental struc- 

 tures, and is, therefore, more primitive and worthy of the greatest 

 consideration so far as it goes. But, it would be a mistake, to as- 

 sume that these problems are so simple that they are to be solved 

 by adherence to any one system of organs, to the exclusion of all 

 others. They do not all possess equal value, but we should, at least, 

 have them in hand, and should use every significant clue and reduce 

 them to harmony, if the development is known one clue should serve 

 to strengthen another. What is now wanted is the coherent history 

 of the different cephalic structures that exhibit a segmental arrange- 

 ment. It is not sufficient to follow the traditional method of describ- 

 ing one stage, and, then, a stage "a little older", and filling in the 

 intervening gap with inferences founded upon observations of the con- 

 ditions in the two forms. What is absolutely required is that 

 embryologists shall have stages close enough together to give an un- 

 broken history. They must possess more complete series, and shorten 

 the time interval between the stages studied. Beard says: "In my 



