METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 13 



blastodisc stage through the position of certain granules. As 

 Hoadley found a lack of localization of future structures in un- 

 incubated blastoderms or those incubated two hours it appears 

 that it must be conceded that some physiological differences 

 underlie the subsequent localization of organs along the axis. 



The nature of the head process has been a subject of discussion. 

 My observations indicate that the head process stage does not 

 differ as regards metabolic conditions from the primitive streak 

 stage and thus support the conclusion of Lillie (p. 82) that the 

 head process is simply a further development of the anterior end 

 of the primitive streak. The region of high metabolic activity 

 at the anterior end of the streak develops into the head process. 



In subsequent development this region retains its high rate 

 of activity and becomes the anterior end of the neural axis. But 

 at about the time of origin of the medullary folds, a secondary 

 region of still higher activity appears. This region is located at 

 the anterior end of the primitive streak (excluding the head 

 process as part of the streak) or in what is known in textbooks 

 of embryology as the primitive knot or Hensen's node. The 

 origin of such a second region of intense activity when one such 

 region is already present can from a physiological point of view 

 be ascribed only to the inability of the latter to control more 

 than a short length of the axis. Beyond that region of control 

 physiological isolation occurs and the origin of a new center of 

 activity is possible. This new center being isolated from the 

 dominance of the original center at the anterior end of the 

 embryonic axis behaves as isolated regions always do when they 

 are intrinsically capable of such behavior it proceeds to the 

 formation ot new individuals, i.e., segments. The experiments of 

 Bellamy ('19) on the frog constitute the best evidence that we 

 have of the applicability of this interpretation, originally derived 

 by Child from experiments on lower invertebrates, to the verte- 

 brate embryo. Bellamy showed that in the frog embryo the 

 distance between the two centers of high activity can be altered 

 by the action of external factors. As the anterior center of 

 activity of the chick embryo differentiates into nervous tissue 

 it is able to dominate more of the axis; the secondary active 

 region, localized in the anterior end of the primitive streak 



