14 LIBBIE H. HYMAN. 



thereupon moves backwards, or more strictly speaking is trans- 

 ferred to more and more posterior cells, leaving incomplete 

 zooids or segments anterior to it. Finally the anterior end 

 dominates the posterior center of activity to such an extent that 

 it is no longer able to carry on independent processes and segment 

 formation thereupon ceases. 



How much of the embryo arises from the primitive streak is 

 a question of considerable interest to embryologists. The answer 

 to the question will of course depend on what is meant by 

 primitive streak. If following the usual usage we regard the 

 primitive streak as that portion of the early embryonic axis 

 including and posterior to the secondary center of activity, that 

 is, the primitive knot, the answer can be given on a priori grounds 

 from the argument in the preceding paragraph as follows: the 

 primitive streak gives rise to all of that part of the embryo which 

 is segmented, that is, from the first somite posteriorly. This 

 conclusion agrees with the experimental results of Peebles ('98). 



One of the most striking facts that has appeared in this 

 investigation is the high susceptibility of each region where 

 development is progressing rapidly. This indicates that intense 

 metabolism is a necessary condition for active development. 

 According to my observations such developmental processes as 

 the closure of the neural folds, elevation of the amniotic folds, and 

 formation of the body and brain flexures are the result primarily 

 of cell activity and not a consequence of mechanical conditions 

 as supposed by early embryologists. It further appears from my 

 observations that intense cellular activity precedes such develop- 

 mental processes. This was clearly the case for instance as 

 regards the closure of the medullary folds and the turning of 

 the embryo. The increased activity in these processes is evi- 

 denced before the morphological change is initiated. 



From my studies of the metabolic gradients in the embryos of 

 the teleosts ('21), the brook lamprey ('26) and the chick and the 

 work of Bellamy on the frog ('19) it seems justifiable to conclude 

 that the formation of two centers of high activity is the regular 

 mode of development of vertebrates. Two such centers have 

 been found in all vertebrate embryos so far studied and they are 

 always located in the same position with respect to the future 



