390 NOTES ON THE BREEDING HABITS 



adult. The embryo begins to rotate in the egg-capsule very soon 

 after the appearance of the medullary folds, so that, unless obser- 

 vations are made at the very first appearance of the folds, the 

 results will be falsified, on account of the rotation of the embryo 

 from its original position. The eggs of the tree-frogs are espe- 

 cially good for experiments such as these, on account of the 

 rapidity with which they develop, decreasing therefore the possi- 

 bilities of a secondary change in position of the egg after it has 

 come to rest, and its plane of division marked. I think it would 

 be possible, by keeping the eggs in a warm room, to cause them 

 to develop the medullary folds within twenty-four hours after the 

 eggs are laid. 



VI.— Enclosure of the Light Pole by the Dark Pole.— In 



studying a series of eggs from the segmentation period to the 

 formation of the blastopore, the so-called overgrowth or epibolic 

 growth of the black cells has been observed. I am quite sure, 

 however (except in the immediate region on the dorsal side of the 

 blastopore, and later over its whole extent), that the yellow cells 

 disappear from the surface, not by an over-growth of the first- 

 formed black cells, but by a process of sp/itti?ig off of cells frot?i the 

 upper corner of the yellow cells themselves. In other words, there is 

 not a general migration of black cells, but each remains approxi. 

 mately in the position in which it was first formed, and new black 

 cells are continually added at the periphery of the black cap by 

 the splitting off of cells from the upper ends of the yellow cells, 

 so that Balfour's sentence that the disappearance of the yellow 

 cells " is effected by the epiblast growing over the yolk at all points 

 of its circumference," is somewhat misleading. As a corollary to 

 what I have said, it follows, of course, that there is a continuous 

 formation of new pigment taking place at the periphery of the 

 black area within the new cells that are being formed^ and also withi?i 

 the ends of the yellow cells which go to fo7'm the new cells in this 

 region. I have not studied with sufficient care the gradual turning 

 in of the cells around the rim of the blastopore. In one living 

 egg, however, I saw, in the dorsal region of the blastopore, some 

 of the cells forming the floor of the archenteron gradually disappear 

 within the blastopore. 



