AND EMBRYOLOGY OF FROGS 389 



sent an ideal to which the egg never attains. The division of the 

 sixteen cells into thirty-two does not conform to any rule, although 

 again, but in a less degree, Ecker's figures may be taken to repre- 

 sent, in the most diagrammatic way possible, the planes of cleavage. 

 Without figures it is impossible to describe the precise method of 

 segmentation ; those of Rauber approximate, I believe, most 

 nearly the truth. In general, we may 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 eight (black) cells 

 are arranged in almost perfect bilateral symmetry, and not a radial 

 one, as given by Ecker. 



V. —Orientation of the Egg.— The relation of the first plane 

 of segmentation 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 a small 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 in a 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 afterwards 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 corres- 

 ponded 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 



