1891.] Embryology. 757 
Say that up to the eight-celled stage the segmentation is very regular, 
but that after that no particular plane of division can be prophesied 
for any segment. Often during the sixteen-celled stage the upper 
sight (black) cells are arranged in almost a perfect bilateral symmetry, 
and not a radial one, as given by Ecker. 
5. Orientation of the Egg.—The relation of the first plane of segmen- 
tation to the adult has attracted a great deal of interest during recent 
years. The relation found in the frog’s egg has been already studied, 
with varying results. Newport’s experiments in 1851, ’53, ’54, are, I 
think, the most to be relied upon, and during the present spring I 
have had the pleasure of verifying his results on asmall scale. The 
eggs of the tree frog were used in the experiment. The outer layers 
of the jelly were removed from an egg which had not yet divided or 
had only undergone the first cleavage. A small, triangular piece of 
card-board was then cut out,and a drop of collodion placed on it. 
The egg with its thin layer of surrounding jelly was placed on the drop 
of collodion as soon as the latter began to stiffen, and card-board and 
egg were then immersed ina dish of water. With a pencil a line was 
drawn on the card-board corresponding to the plane of first division. 
The water was changed several times until all trace of ether was gone, 
and afterward set aside in a quiet and warm place. Several other eggs 
were prepared by the same process. At the end of forty-eight hours 
the medullary folds began to appear, and it was then seen that the 
plane between these corresponded exactly, in most cases, to the plane 
indicated on the card-board, and therefore the obvious conclusion is 
drawn that the first plane of division divides the egg into two parts, 
corresponding to the right and left halves of the adult body. In 
a few eggs the first plane was somewhat to the right or left of the 
mid-line of the adult. The embryo begins to rotate in the egg- 
capsule very soon after the appearance of the medullary folds, so 
that unless observations 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 especially good for experiments such as these, on account 
of the rapidity with which they develop, decreasing therefore the 
possibilities of a secondary change in position of the egg after it has 
come to rest and,it 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. 
6. Enclosure of the Light Pole by the Dark Pole—In studying 
‘a series of eggs from the segmentation period to the formation 
