THE ORIENTATION OF THE PROg's EGG. 377 



believe that this is undoubtedly so, and as yet I have seen no 

 indication that would tend to a contrary conclusion. 



It has been noted that with the splitting off of cells from the 

 upper corner of the yellow cells of the lower hemisphere new 

 ectoderm-cells are formed, and it had been generally supposed 

 that with this growth a continuous formation of pigment took 

 place, the black pigment gradually growing down over the 

 whole egg. I have found that the growth of pigment does not 

 in any way correspond to the growth of new ectoderm-cells, 

 but, on the contrary, there seems to be a great variation in the 

 amount of pigment found in various lots of eggs of the same 

 stage procured at different times. Some eggs of the two- or 

 four-cell stage have the pigment covering more than two thirds 

 of the egg, while others at this point of development have only 

 a black cap of pigment extending down to the third furrow. 

 However, all the eggs of the same stage from one cluster, and 

 hence laid by the same frog, are alike in the quantity of pig- 

 ment. The amount of pigment in the egg seems a variation 

 dependent on conditions previous to the beginning of segmenta- 

 tion, and due to individual difference in the adult frog. There 

 is, of course, some formation of new pigment in the later seg- 

 mentation stages, and a most marked and rapid change in the 

 amount of pigment formed at the time of the first appearance 

 of the blastopore. 



In all the eggs, from the earliest stages up to the blastopore, 

 there is one marked peculiarity of the pigment. There is 

 always a greater deposit of pigment on one side of the egg than 

 on the other; and if we judge the exact position of the lower 

 pole from the crossing point of the first two segmentation 

 furrows, we find that the pigment is not only denser, but it 

 comes down much nearer to the lower pole on one side than on 

 the other. As this was found to be the case in greater or less 

 degree, and in some eggs very markedly, in all the stages up to 

 the beginning of the blastopore, and, moreover, in eggs procured 

 at several different times and places, I judged it could not be 

 an accidental variation. 



I have tried, therefore, to find out — 



