238 EICHARD ASSHIITON. 



blastopore concresce as Roux assumes^ then marks made upon 

 the lips should show upon the dorsal surface somewhere along 

 the neural folds. In no case did I ever find this to occur. As 

 the result of such injuries, I either found the scar upon the 

 lateral lip of the blastopore when completed, or else as in figs. 

 6 a, 6 b, further removed from the blastopore but in the same 

 relative positions. 



I never found a defect in the neural plate except when the 

 dorsal, or near the dorsal lips of the blastopore, was injured. 

 This spot is unfortunately at the same time the most interesting 

 to injure, and the most delicate, and most liable to produce 

 abnormalities which prove very little. There is certainly no 

 need to assume a concrescence, as the facts can be equally well 

 accounted for by other means, which to my mind agree far 

 better with the development of other Vertebrates than does the 

 concrescence theory. 



In other words, I believe that as in the rabbit, so in the frog, 

 there is evidence to show that the embryo is derived from two 

 definite centres of growth, the first, and phylogenetically the 

 oldest, being a protoplasmic activity which gives rise to the 

 anterior end of the embryo (= gastrula stage) ; the second, 

 which gives rise to the growth in length of the embryo : which 

 centres of growth occupy the same relative positions in loca- 

 tion and in sequence of time, and probably to each are due the 

 same parts of the embryo. 



In the rabbit the area is a spot which assumes for a time a 

 linear form, but is unaffected by its change of shape in the 

 functions it has to perform. 



So in the frog, although at first crescentic, then circular, 

 then linear, and ultimately a knob, its function is precisely 

 the same as in the rabbit, and is unaffected by the change in 

 form. From the moment of its first appearance it performs 

 its one function — that of adding on new cellular units to the 

 previously existing embryo. 



One difference of effect is that owing to the manner of its 

 coming into existence, one portion arising before the other, 

 that portion — the dorsal — becomes functional before the 



