380 T. H. MORGAN AND UM6 TSUDA. 



in this case would be as near to the first furrow as to the 

 second (Fig. c). It will be seen on examining the above dia- 

 grams that the apparent variation of pigment in the three 

 cases depends on very slight differences. A little shifting of 

 the pigment or an addition to one side or the other would 

 change Fig. a to Fig. c or b. Judging by numbers, a would 

 seem to be the more typical one. 



One thing remains obviously unchanged in all the eggs. 

 Of the four cells into which the egg is divided, one cell, a, has 

 always the greatest amount of pigment ; and cell b, which is 

 opposite to it on the other side of the egg, is the lightest. 

 This is true in every case, and these two opposite sides can be 

 distinguished in eggs far advanced up to the end of seg- 

 mentation. The cells marked c and d are intermediate, being 

 neither so dark as a nor so light as b. 



The pigment on the darker side of the egg not only extends 

 much farther down, but I have observed on dissecting the egg 

 in the upper hemisphere that in some cases the pigment 

 extends inward almost to the centre of the egg (blastula), and 

 to about twice the depth of the opposite side. 



The interesting point in connection with the two opposite, 

 the darker and the lighter, sides of the egg, on which I have 

 dwelt at such length, is that the less densely pigmented half 

 of the egg very early in the segmentation shows signs of a more 

 rapid development and growth than the darker and pigmented 

 side. This is true of the cells of the upper hemisphere as well 

 as the lower. Moreover it is on the side that shows this 

 advance in development that the dorsal lip of the blasto- 

 pore makes its appearance. 



I first noticed the unequal growth of the cells in an egg of 

 about ninety-six cells, sixty-four in the upper and thirty-two 

 in the lower half. There was a decided retardation of one side 

 of the egg. In later stages the lighter half seems often two 

 stages ahead of the other side. Figs, iv, v, show the unequal 

 development in the early stages. Figs, iv, v (camera draw- 

 ings), are surface views of an exceptionally fine egg, in which 

 the unequal development is seen to what might appear an ex- 



