THE ORIENTATION OP THE FEOG's EGG. 383 



the line of pigment on one side, as we should expect, while the 

 blastopore on the opposite side is less than one third of the 

 distance from the lower to the upper pole.^ 



It has been almost conclusively proved by previous ex- 

 periments and observation that the plane of the first furrow 

 in the case of the frog divides the egg into halves correspond- 

 ing to the right and left sides of the embryo ; and this study of 

 the blastopore does not contradict, but would tend to confirm 

 the fact. Although the arc of the blastopore is often not 

 opposite the centre of the crescent of pigment (as it is in 

 fig. xii), this is easily accounted for by the distribution of the 

 pigment as shown in Figs, a, b, c (text). If we suppose the 

 second furrow rather than the first to cut through the centre 

 of the crescent (Fig. b) , we should have the pigment much as in 

 fig. XI, allowing for the formation of some new pigment. 



III. 



The eggs of two species of frogs were used for the ex- 

 periments recorded below. Eggs of Rana temporaria were 

 found on the morning of March 25th. These had not as yet 

 segmented. The eggs of another species (not determined) 

 were brought to the laboratory on April 4th. These had just 

 begun to segment. Since much of the experimental work was 

 done on these eggs, it was first necessary to find out whether 

 the facts recorded in the last section were also applicable 

 here. 



A study of representative stages showed the same distribu- 



' It has been thought by some investigators that the blastopore formed 

 much higher up in the egg, but it needs only a superficial study to show that 

 this at least is impossible. The cells within the blastoporic ring are non-pig- 

 mented and yolk-cells. A study of the surface view of the early stages shows 

 that the pigment from an early stage often extends down on one side two 

 thirds of the side, and on the other one half of that side. The blastopore 

 forms lower down than the pigmented area, and this would make it at least 

 halfway down the egg from the upper pole, and much below the plane of the 

 third furrow. We see in fig. xiii that if the lower pole, as I have attempted 

 to show, is marked by the large cells the blastopore appears below even the 

 equator of the egg. 



VOL. 35, PART 3. NEW SEB. D D 



