METABOLIC (iR AIMKNTS OF VERTEBRATE EMHRVOS. 15 



embryo one at the anterior end of the antero-posterior axis and 

 the other in the axis at a more posterior point. The posterior 

 center is the dorsal lip of the blastopore in cyclostome and 

 amphibian embryos (probably also in amphioxus), the posterior 

 end of the embryonic axis in teleost fish, and the primitive knot, 

 subsequently the "tail bud," in the chick. This posterior center 

 of activity is like a growing point which growing backwards 

 deposits the trunk of the embryo anterior to it. It is interesting 

 to note that Assheton ('94a, '946) long ago recognized in the 

 embryos of the frog and the rabbit the presence of these two 

 centers of activity and correctly identified the posterior one as 

 the dorsal lip of the blastopore in the frog and a region about 

 corresponding to the primitive knot of birds in the rabbit. 

 He called these regions primary and secondary centers of cell 

 proliferation. He stated that the primary center forms the head 

 of the embryo anterior to the first somite, while the secondary 

 center forms the rest of the embryo. My ideas here presented 

 entirely coincide with these statements of Assheton, which seem 

 to have been generally overlooked. Eycleshymer ('98) working 

 with the amphibian embryo accepted Assheton's ideas. He 

 showed that the primary center, the center of the animal pole, 

 becomes the anterior end of the embryo, the dorsal lip of the 

 blastopore the trunk. He considers these to be two regions of 

 high activity, agreeing with Assheton. He stated: "The primary 

 area of cell activity at the upper pole of the amphibian egg forms 

 the basis of the cephalic end of the embryo." 'The secondary 

 area of cell activity on the blastoporic side of the egg forms the 

 basis of the greater portion of the posterior half of the embryo." 

 (Eycleshymer entertained the probably erroneous notion that 

 the extreme posterior end of the amphibian embryo was formed 

 by concrescence.) Adelmann ('22) and Kingsbury ('24) seem to 

 agree with the conception that the formation of two centers of 

 high activity is the regular mode of development among chor- 

 dates. Adelmann's idea that these two centers arise by the 

 splitting of one center is, I think, incorrect. It does not agree 

 with the actual facts of observation by the susceptibility method 

 on several types of vertebrate embryos. The posterior center 

 arises independently of and usually at some distance from the 



