THE ORIENTATION OF THE FROG's EGG. 403 



structure is the large archenteron that begins at the edge of 

 the black in the mid-dorsal line, and extends as far forwards 

 as the level of the suckers. The medullary folds in these 

 embryos have not as yet rolled in to form the (half) nerve- 

 cords. Longitudinal section of the embryo drawn in fig. xix 

 shows that at the ventral lip of the blastopore only a very 

 slight depression is present. 



Conclusion. — His supposed concrescence of the Vertebrate 

 embryo to take place by apposition of the sides of the germ 

 ring, and due to this process the embryo grew posteriorly. 



Balfour believed this to be untrue, and that the posterior 

 end of the embryo grew in length by a process of intussuscep- 

 tion in front of the last segment of the body. 



Roux's experiments by sticking the border between black 

 and white point directly to a process of concrescence of some 

 sort. If Roux^s experiments are accurate we must suppose 

 that the cells that will later form the central nervous system 

 are already laid down along the black-white border. These 

 cells must come to the middle line as the blastopore gets 

 smaller. The closing of the blastopore from before backwards 

 would then be due not to a backward extension of all of the 

 material of the dorsal lip over the yolk, but would 

 take place by new tissue coming up to the middle Hue from 

 the sides and placing itself with or behind the cells already 

 present in the dorsal lip. 



I should not regard this, even if it took place, as apposition 

 in the strict sense of the word, nor is it intussusception in 

 Balfour's sense. It would not be intussusception because 

 new tissue is coming in continually from the sides to mingle 

 or mix with the cells already present and multiplying in the 

 dorsal lip of the blastopore. Nor would it be apposition in 

 His's sense, because the lateral borders of the blastopore are 

 not laid down side by side, since the blastopore does not close 

 by actual apposition of its lateral rim. I shall not here 

 attempt to formulate a theory of overgrowth, but merely to 

 point out the apparent bearing of the evidence furnished by 

 this experiment of Roux. It will, I think, be possible to de- 



